Re: Larry's 1951 Club Sedan Project

Posted by Larry51 On 2010/7/1 7:35:10

Cavity Sealing, Rust-proofing, Priming and Painting

Because rust had done quite a lot of damage to the Coupe over its 50 years of life I had to think long and hard about which of the many different products and combinations of primers, undercoats, sealants and cavity waxes etc that could be used.

I wanted to gain the absolute best protection for the car, and cost or hours required was not a big consideration. It is not an easy thing to decide which anti-corrosion and paint systems to use, as there are dozens of supposedly good products available.

Some issues that I needed to consider were -

- Toxicity of primers and paints (isocyanates in 2-pak systems are very toxic, and special care is needed even when sanding back dry coats). I want a DIY product as I'm intending to do all surface prep and spraying myself. It might not be a 100% finish but cars I've sprayed over the years have probably been around the 90% level, and it is really enjoyable to know the final 'gleam' of the finish is something that you achieved yourself.
- Did I want to stay traditional and use the finish originally used (acrylic), or make use of tougher systems like 2-pak?
- Would I be able to achieve really good results easily if the need arose for re-painting the Coupe's paintwork at some future time? Modern 2-pak paint may prove to be more problematic if fixing a scratch or repainting a few small areas in a year or three.
- Cost of product versus results. Some finishes are four times the cost of others . . .
- Whether to use 'permanently tacky' cavity wax or a rust preventive that will form a dry film. Which is best? The tacky ones will attract dust but work extremely well and last a long time.
- Should I use the traditional 'dum-dum' joint sealer, or a more modern type like a polyurethane??
- How about stone-guard coatings - are the bituminous compounds better than others?
- Should I use epoxy primers etc under stone-guard coatings and on some metal (like floor pans), or go POR-15?

Well, I'm slowly working through these choices, and trying to make sure that I don't choose some incompatible mix of products that will give poor results.

Having seen a good paint job lift off the metal due to a failed etch primer makes me a bit concerned about adhesion of paint systems. I reckon the biggest problem you can encounter (aside from rust forming under a coating) is poor adhesion of paint. So my aim is to get the best adhesion so there will never be a problem in that regard.

So far the 'winning' primer for external surfaces is the PPG Epoxy-Urethane 2-pak primer. It's dear as poison (- probably because it is poison!). It has to be applied in specific ways, and costs a bomb but has great rust prevention and adhesion.

One of the limitations for this PPG primer is that the final paint (base coat) has to be applied within hours of the primer being applied. Otherwise a second coat of the primer needs to be applied just before the colour coat. So if you have your Packard sitting around for a few months or years in primer, it has to be rubbed back and primed again just before final painting. This is not the case with many other single-pak primers.

A problem I have had more than a few times is the quality of information that you get from the technical 'experts' when you call the Helpline of a paint or coatings company. Ring twice and you'll probably get two different answers which is a concern.

For example, when speaking to one 'expert', I told him the car had been grit-blasted and some areas had been primed (with rich zinc primer) but other areas had not been primed yet. According to this person, once a car has been blasted, you need to get it primed within a few hours, or any subsequent primer / paint etc will just trap moisture under it and the metal will rust badly. Well, most automotive paint companies say that is not correct. When you apply their particular primer over any bare metal then there is no chance of rust forming provided the surface was prepared properly.

So technical support often leaves a bit to be desired.

I'm sure that properly prepared bare metal will not rust after a good coat of primer and colour is applied over it, despite sitting around for months after grit blasting. But of course some surface rust is likely to form after a while (depending on climate etc), and this has to be totally removed and the surface treated (Phosphoric acid).

I have been protecting bare metal with a lanolin surface spray which seems to work very well. No rust has formed at all with this coating over the bare metal. The lanolin can easily be removed with water followed by thinners etc.

I still have many decisions to make, but I've made a couple of choices already, having settled on painting the entire underside floor pan with POR-15 (after properly treating with Metal Ready). And various mudguards (fenders), trunk lids etc are getting the PPG Epoxy-Urethane primer even though I'm leaning towards using traditional acrylic as the topcoat, rather than a 2-pak finish. At least I will have the opportunity to go 2-pak if I decide to, by having used the right primer for 2-pak.

The final paint job is still a long time off into the future. Will post about the products chosen later . . . .