Classic cars forum & vehicle restoration.

[Kelsham]

Dave is correct, I have had serious sinkage in the past when using cellulose.

Regards Kels.

[Bitumen Boy]

Well, that's not a problem I've had/noticed, but there are a few possible reasons for that. My techniques aren't exactly conventional, they work for me but they really wouldn't suit everybody...

1. I don't spray. Never got on with it. I'm normally using an oil base primer, and even if I do use cellulose - like the Etchweld - it still goes on with a brush. Usually, then, that's a coat or two of primer over the finished repair, a practical minimum of three undercoats, then two or three colour coats - maybe also a couple of barrier coats and 2 or 3 of stonechip on top of the primer. All fairly thick, all applied with a brush - it can hide even filing marks on the filler so I probably wouldn't even notice any sinkage...

2. When I say good results, perhaps I should have qualified that - a good result to me is if the rust hasn't come back after 5 years, for my own cars I don't consider a perfect finish and an absolutely invisible repair to be enormously important.

3. I take a long time to do most jobs, and I don't mean in a good way. Primer can take days to go off in the winter, I don't feel like going up to the garage every day, often I will find something more interesting to do when I get there! Certainly filing/sanding is almost never done the same day. Maybe shrinkage is less of an issue if finishing is left for a while?

I guess the only way to tell for sure is if someone - not me! - was to experiment on some scrapyard panels or similar. Apply two lots of filler, one over bare steel and the other over primed steel, apply topcoats, polish up and leave both in the yard for a year, or two, or three... see which fails first. Anyone fancy the challenge?

[ukdave2002]

Going back to Julians original question

Ideally filler should be directly on metal, the metal should be repaired and worked so as the thinest amount of filler is needed, then keyed to aid filler adhesion, cleaned to remove anything but bright steel, treated to a splash of phosphoric acid to neutralise any corrosion, and cleaned with a solvent panel wipe.

Etch prime performs 2 functions; firstly to provide a better key to the metal, secondly to neutralise any minor surface corrosion (I'm not talking about rust that can be seen by the naked eye!! that needs to be removed completely first). So by keying the metal where filler is required and treating with phosphoric acid we have replicated what an etch primer would have done. Its also fine to spray etch primer over filler.

In the pic below my MGA looks full of filler however none of it is more than 1mm thick, it was then etched primed followed by a couple of coats of high build primer and then stopper to deal with smaller imperfections that could only be seen after a dusting coat.


This was the prep that was done to ensure only a skim of filler was needed, the rear end had taken a quite knock at some point, there was filler 3/4" thick in a couple of places that was removed!! .

[Julian]

Thanks dave, that's got me able to push on with the job knowing I'm not going far wrong!

Just another question, how to you find 2k primer - pros & cons etc?

I've been using 2k top coat now for a few years and would never (willingly) go back to 1k. But I still use cellulose high build primer because with a warm environment I can be sanding it within 5 minutes or so of spraying so the process of getting a good surface for top coat is fast and easy.

Cheers Julian.