I had no welding repairs to do on my own chassis and so am unable to give first-hand advice on such repairs. Many chassis survive intact thanks to the heavy gauge steel used. Likely problem areas are at the rear: Underpan, cross-member (including spring shackle mounts), and the side members where the ply panels bolt onto them and trap water. There was some pitting to my side members, and since this creates water traps, these should be filled. A suitable epoxy metal filler should suffice assuming that the steel is still thick enough to be structurally sound.
Painting the chassis - and inner panels, brackets and so forth - presents a confusing topic. Paint and painting is a complex subject and in-depth articles tend to be too scientific. Practical advice can also raise confusion as there are so many options and mistakes creep into articles. Worse still, paint retailers and makers market their products according to intended use, with vague subjective claims as to the paint's performance, and little word about what type of paint it really is.
I hope that the following article will clear up the issue in the context of classic car restoring, without attempting to make this a comprehensive treatise on paint.
Paint is made up of a number of components, but only two of these are of interest to us when choosing a paint: Resin (AKA medium or varnish) is the main liquid part. Pigment is the solid part ground into a fine powder. Each provides certain qualities depending on what types are selected. A lot of debate about paints dwells on the rust-inhibiting qualities of certain pigments, but the primary concern is getting the paint to stick to the metal, and the choice of resin may be more important. Final choice comes down to one's circumstances and personal preferences.
Red lead: This has good rust protecting qualities, as does white lead. It is largely discontinued (except for historic buildings) due to its poisonous nature.
Red oxide: This is one of the iron-oxide pigments. It has no particular rust protecting qualities (although additional anti-corrosive pigments might be added to a red oxide paint), but protects the paint against UV light deterioration. However, it should not be dismissed for use on steel depending on the type of resin it is carried in.
Zinc chromate: Note that this has nothing to do with galvanising protection, which requires metallic zinc. Zinc chromate has a greenish-yellow colour and is commonly used as pigment for protective primer paints for aluminium. It also has rust inhibiting qualities for steel. There are health issues with using this substance and so guidelines should be checked out before using this type of paint. It has largely been superceded by alternative pigments carried in 2-part epoxy resins.
Zinc phosphate: This too, has nothing to do with galvanising protection for steel, but it does have rust inhibitive qualities. It is more usually used as part of chemical conversion coatings. These are applied to prepare the steel for painting. Mixed with diluted phosphoric acid and other ingredients, it converts any remaining traces of rust to iron-phosphate and also creates a non-conducting layer on the steel. This inhibites corrosion and gives a surface that paint will more readily adhere to.
Zinc rich (zinc powder): This pigment will provide galvanic protection to steel provided that it is in good electrical contact with the steel surface. That makes it less viable for DIY painting projects, because the steel surface needs to be blast cleaned to a high standard, especially if the zinc powder is carried in an epoxy resin. There is an added problem when brushing on zinc-rich paint: The weight of the pigment may cause the paint to fail to stick unless the correct brushing technique is used. For those two reasons, I decided not to use zinc-rich primer myself. If you do use it, be sure not to use any conversion coatings as a preparation, since these coatings reduce the conductivity of the metal, but zinc-rich paint needs the opposite. Zinc powder is usually carried in a resin of epoxy or silicates.
Aluminium: Aluminium flakes in paint help to protect steel. They are also used for high temperature resisting paint. For the AC, it is ideal for wood primer, and in my experience, makes the best primer for wood by far.
Alkyd: These resins are combined with oil in varying proportions depending on the intended use of the paint. They don't have a good resistance to acids and alkalis, and so are not good for the undersides of cars that are exposed to road salt. However, some formulations are designed to improve this performance and there are chassis paints available that are alkyd based. A lot of paints for decorating are alkyd/oil based as are many so-called synthetic paints. I think that terminology came about before many other types of synthetic resins came into use.
Inorganic silicates: These are usually used with zinc powder pigment and are very hard wearing and high temperature resistant.
Epoxy and epoxy mastic: These are two-part resins that have to be mixed together in the required proportions. They are excellent for primer paints because epoxy adheres very well to steel that has been hand cleaned (or blast cleaned). They are very hard wearing, do not let moisture through and also resist solvents. This is one of the best resins for rust protection. The downside is that they deteriorate in ultra-violet light and the appearance (shine and colour retention) is inferior to other paints. Epoxy mastic - or modified epoxy - will have more specific attributes for specific applications. "Jotun" produce such an epoxy mastic, branded as "Jotamastic", used for boats, offshore installations and other harsh environments. They do a few variations of this paint. Significantly, it is compatible with single part paints, so you can paint over old paint or touch up, and can choose what top coat to finish with.
Polyurethane: This is available as either a 1 part or 2 part paint. 1 part polyurethane is hard wearing and ideal as the top coat for the AC's wood frame. 2 part polyurethane is even more hard wearing. It is used for garage and workshop floor paints and seems almost indestructable. It also gives a good, deep shine, good colour retention, and resists UV light, making it an ideal top coat for chassis and inner panels. This is my personal choice to use over an epoxy primer. It is readily available from marine suppliers. Note that 2 part polyurethane can only be applied over suitable 2-part primer paints such as epoxy.
Bitumen: Known as "asphalt" in the USA, it is water-proof and protects against rust. It also has sound-deadening qualities and car makers (including AC) used to paint the interior of the passenger compartment, wood frame and alloy panelling, with bitumen. It was also used as a sealant with damp-proof course to seal the wood frame to the chassis. AC made extensive use of bitumen to stick interior trim into place. Unfortunately, this bitumen paint dried out after a few years and trim peels off. It is also not a good idea to use it on woodwork (at least on a car frame) because it is water-proof and does not allow the wood to "breath". It also has poor solvent resistance.
Bitumen is also used in car underseal, along with rubber. In practice, underseal has often proven to be very poor for rust protection unless its application has been done meticulously. I would not recommend its use on the AC.
Cellulose: Ever since car bodies have been spray painted, nitro-cellulose was used until more modern paints took over. The AC's external bodywork was sprayed with cellulose. It is less suitable for brush painting because it dissolves in its own thinners, and so a second coat would normally ruin the first coat. Spraying works because the solvent evaporates fast and the paint dries quickly. It is not suitable for the car's underside, and even for the bodywork, has poorer durability than more modern paint systems.
Stoving enamel: Not to be confused with "synthetic enamel" or other glossy paints that one might purchase. Stoving enamel is applied in industry to create a much harder wearing surface finish, and involves heating the paint to set it.
Two pack acrylic: Also confusingly known as "two pack" or "2K", two pack acrylic took over from cellulose as the standard paint system for exterior bodywork. It requires a suitable paint-shop with breathing apparatus (fumes are poisonous) and therefore is not suitable for DIY use. It has since been largely superceded by water-based paint systems to cut down on air pollution.
POR15: This is a specialist paint that cures by absorbing moisture from the atmosphere, curing faster in damp conditions. Like epoxy, it is non-porous. I haven't used it myself but it is reputed to be extremely hard wearing and resists acids. Like epoxy, it is vulnerable to UV light, but POR15 do a suitable top-coat for car chassis. It is not compatible with other paints unless a barrier coat is used. Being so hard wearing means care must be taken not to get it where it may cause problems, such as in threaded holes. It also needs POR15's own preparation solution prior to application. Wear gloves for protection while working with this paint system. Some user feedback says that certain old paints may be damaged by the prep solutions, so beware if you are doing touching up with this system. Like any paint system, the maker's preparation procedure must be followed.
This is an industrial process, but is available to DIY restorers who can fully strip the chassis and transport it to a powder-coating plant. A major downside to this process is that if paint does get damaged (although the finish is hard-wearing), it is difficult to touch up.
I know this isn't painting, but it is relevant to this topic. The term "galvanising" is usually applied to hot dipped zinc coating (dipping in a very hot zinc solution) rather than electro-plated zinc, even though the latter also provides galvanic protection. Electro-plating gives a thinner, but more even coating, suitable for screw threads. Galvanising gives a thicker, more protective coating. Galvanic protection means an electrical process that occurs when two different types of metal (or carbon) are in contact with each other and water (preferably acidic water). One of the metals will corrode in preference to the other metal that is therefore protected. Zinc corrodes in preference to steel, and the corroded zinc powder formed also helps to keep out moisture. As with most things, the quality of the galvanising is important. It can be painted over, as long as this is done properly, as paint does not adhere well to shiny zinc. The high temperature involved places limitations on what can be coated by this method.
Preparation for painting steel
Preparing the chassis for re-painting will most probably consist of the following steps:
De-greasing is vital prior to painting and it is best to purchase something from your paint supplier. Whatever you do, don't use paint thinners for pre-paint cleaning. Solvents cleaners are good to help with the initial stages of cleaning.
If you are removing AC's original paint, it probably includes lead-based paint with its well known health hazards. It is preferable to use chemical paint-stripper and then dispose of the waste according to local authority regulations. Note that 60 year old paint does not strip off easily! Some "elbow grease" is required. Small brackets can be cleaned using kits based upon industrial methods such as electrolytic stripping. These will strip paint, grease and rust all in one go.
A lot of the steel brackets and inner panels were coated in bitumen and nothing else. It can be scraped and sanded off. If sanding is not fully successful, then remember that bitumen has a low resistance to solvent cleaners and acids. I found that any bitumen I had failed to remove, was dissolved by the phosphoric acid from the phosphating treatment, prior to painting, allowing me to wipe it off.
Mechanically removing remaining visible rust:
For a home DIY restoration, one is likely to use a rotating wire brush driven by an electric drill - at least on anything large like the chassis. It is vital to wear eye-protection since wire bristles will fly off every so often. A problem with wire brushing is that any traces of grease may get ground into the steel, and you end up with a grey glazed over appearance. Rub it with emery-cloth, and you may find more rust under that glaze. Change brushes frequently to reduce this problem, and alternate between wire brushing and abrasive cloth. Also note that wire brushing leaves a glazed finish that paint might not adhere to, so always use abrasive cloth/paper after brushing.
A conversion coating for steel should achieve 3 objectives: Firstly, converting the last traces of rust to render it inert. Secondly, to passivate the surface of the steel, making it non-conducting (electrically), to make it slower to rust again. Thirdly, to create a surface that the paint sticks to well.
A good approach to this is to apply a phosphate coating, which is mostly made up of zinc-phosphate and diluted phosphoric acid. No need to brew your own mixtures, as a good paint supplier will have a suitable solution available.
Preparation for painting aluminium alloy
The aluminium-copper alloy (as also used on aircraft) employed on the AC, has a low rating for corrosion resistance compared to pure aluminium and other alloys. Therefore, it should be painted for protection wherever it is exposed under the car or under its wings.
Aluminium oxide layer:
Bare aluminium quickly develops an oxide layer that is clear and very hard to remove. And if you remove it, it returns within a few minutes.
Avoid cleaning with anything metal such as wire-wool or wire brushes. Metal particles might become embedded and risk causing galvanic corrosion.
As with steel, every trace of grease must be removed and all traces of the degreasant then rinsed off (if the instructions call for rinsing). Also, be sure to degrease using solutions designed specifically for pre-paint preparation - or at least avoid using household cleaners.
Etching and chromating:
In the aircraft industry, the oxide layer is often removed chemically, and then the surface is chromated. This gives the best protection, especially for aircraft alloys that are more prone to corroding (as with the AC). It also gives a good surface for paint adhesion.
Unfortunately, the chemicals involved are somewhat dangerous, and require a knowledge of the potential hazards. Non-toxic conversion coatings have also been developed, but all these chemicals are difficult to obtain, and are usually expensive. "Deoxidine 624" is used to remove the oxide layer, prior to applying "Alocrom 1200" ("Alodine 600" in the USA) and then primer paint.
If the above approach isn't practical, then the more common (and cheaper) practice is to spray on self-etching primer. Check compatibility of the etch primer with whatever paint you apply on top of it. Epoxy paints can't be applied over etch primer (ditto for Eastwood's chassis black).
2 part epoxy primers have largely taken over from zinc-chromate in the aerospace world. Epoxies protect the metal even without top coats of paint. However, these are usually used after chromating the aluminium's surface.
Preparation for painting brass
The radiator cowl on the AC is made of brass (except a few of the earliest ACs that used aluminium alloy). The inside will need painting black. Also, the door-step plates have painted black lettering (lettering will need masking when these plates are re-chromed unless you have the later anodised aluminium plates). The original paint on my cowl peeled off very easily, as this is a difficult material for good paint adhesion. Apart from the usual cleanliness of the surface prior to painting, a bit of etching is needed too. However, note that strong acid etching of brass may be dangerous due to the fumes produced. A light spray coating with self-etching primer might be sufficient? Advice on this topic is rather diverse, but extreme cleanliness is always the priority with all types of metal painting.