Epoxy primer cracking

[Pdv]

I have been preparing a rear valence for topcoat, sanded down the original paint as it looked very good and welded the holes for the bumpers and filled any blemishes.

Gave it two coats of primer filler and flated, then gave it a coat of epoxy primer, onlyto find cracks in a couple of places.

The only thing I can think is I gave it an extra pass, but even then on the bench under the light it looked a thin covering in one place so I don't  think it was applied to thickly.

Any thoughts please.

Steve

[Homersimpson]

I'm not sure you should put epoxy on top of filler primer and ideally not even on top of existing paint.  I've always put epoxy on bare metal and then filler primer on that (allowing the epoxy to dry for a week or so before the filler primer).

It may have cracked because there is too much paint on the panel now or because its reacted with what you have underneath it.

In these circumstances its better to take everything off back down to bare metal and start again as you don't want it to happen again once the top coat is on.

[chrishawley]

I'd be thinking along the same lines as to the likely, or at least possible, occurence here: In current ambient temperatures the primer-filler(PF) may be dry enough to sand but not in fact entirely 100% cured (assuming 2k primer from a gun). Such that the thinners in the epoxy have 'woken up' the PF allowing uneven shrinkage and hence cracks.

A rescue procedure for situations such a this is to heat-dry the area then put down a layer of isolator primer and apply PF again over that - but that's probably more effort than simply sanding back and PF'ing again. It might be that in this context the epoxy is not adding much value if not applied to bare metal as the the root coat.

As an aside PFs don't like to be applied to a base that is too finely sanded: 80 grit for example is fine whereas too smooth (e.g. 400g) impairs even adhesion.

[Pdv]

Well, problem solved, I mixed up some Dolphin Glaze filler last night and flated it this morning.

I then mixed up some epoxy primer and realised that what I had been using was primer without the hardener that I had mixed up ages ago, so sprayed on the new primer with hardener and bingo all fine, have given it three coats of primer filler put the valence by the log burner in the workshop which will give a gentle heat overnight.

In my defence I had a head injury that required surgery, a bleed and clot on the brain so when I mixed up the original primer had not put the hardener in with the primer. 

Thanks for the replies all good information to take on board.

Steve

[chrishawley]

Glad you've got a remedy. But I had a comparable experience today:

Job was to epoxy primer a very large panel. Of which some was bare metal, some was good metal with a waft of BondaPrimer, and some corners had filler in with traces of 1k primer and colour that had been used for guide coating. As it was drying I could just see hairline cracking in the 1k areas; nothing tragic and they'll flat out ok, but just a clue that epoxy is not quite as 'unreactive' as might be supposed. 

I'll see how it is in the morning, but maybe the learning point is that epoxy really does have to be applied as the root coat and not on top of other stuff. Hmm.

 

 

[Pdv]

That's interesting Chris, I googled it yesterday and came across something from  Frost Restoration saying that it is good for applying over painted  surfaces. But yes bare metal is the best way to g.

In actual fact I did not have any reaction with the paint on the panel it was me not doing the right thing.