Classic cars forum & vehicle restoration.

colwyn500 · Joined: 21 Oct 2012 Posts: 1745 Location: Nairn, Scotland

The instructions on UPOL High Build cellulose primer indicate that it should be overcoated within 5 hours from spraying. For me that's impossible as it I need several weeks to get round to prepping it all.
Would the time limit refer to the danger of moisture absorption through to the steel beneath? If so would the fact that I have epoxied the whole shell mean I am Ok.?
Or does it "skin" over after sanding?

ukdave2002 · Joined: 23 Nov 2007 Posts: 4119 Location: South Cheshire

No the risk is that the primer will absorb moisture, and when the top coat is applied this will effectively seal the moisture in. Weeks sometimes months later you will begin to notice small blisters in the paint as the moisture makes a bid for freedom, sealed in moisture has ruined many a paint job Sad

As the paint manufacturers have no idea what type of conditions the primed panels will be stored in, they tend to be very conservative with the maximum recommended time, in good dry conditions you will get away with longer, but it's impossible to put an accurate figure on.

Why not just stick with epoxy primer?

Cheers

Dave

petermeachem · Joined: 23 Sep 2013 Posts: 358 Location: Chichester Sussex

I've the same problem. As far as I can tell cellulose primer will stick to epoxy but top coat won't. I can't see any way of top coating within 5 hours, more like 5 weeks. I was going to ignore the advice as it would be unworkable for me anyway.
I'm still at the stage of arranging into a single garage the contents of a double garage and a large garden shed so I can progress the car at all. And do work on the horrific state of the decorations in the new house and garden which is a higher priority. My goal is to get the car painted this summer.

colwyn500 · Joined: 21 Oct 2012 Posts: 1745 Location: Nairn, Scotland

I'm in a very similar situation to you Peter Sad

I needed to prime because I did the filling and flattng after the epoxy..because you can. Epoxy has no real "body" to it so no good for hiding small imperfections.
I think you covered the answer Dave.
Thanks.

ajlelectronics · Joined: 04 Oct 2010 Posts: 168 Location: Gloucester

So the simple answer is just to accept the advice given and use epoxy primer, rather than cellulose primer. You can leave it six months or more, as long as you keep the UV off it. Even then, you can flat back before finishing.
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ukdave2002 · Joined: 23 Nov 2007 Posts: 4119 Location: South Cheshire

When I did the bodywork of the MGA, I worked on individual panels from start to finish, so that every panel went from bare metal to gloss coat in a few days .
Not the most efficient way to work, but I was specifically trying to avoid the problems mentioned.

Cheers

Dave

aps · Joined: 15 Mar 2013 Posts: 37 Location: aberdeenshire

a friend of mine has a major problem with paint blisters .one of his customers has filled and primed his beetle ready for top coat ,the problem is as soon as paint is applied the car is covered in blisters ,so now the car has to be stripped and started again from bare metal to achieve a good result .wayne aps

colwyn500 · Joined: 21 Oct 2012 Posts: 1745 Location: Nairn, Scotland

Epoxy is really resilient and impervious but would be hard work used as the primer before topcoat unless you have already produced an immaculate car body. I used it to give me time to straighten everything out with filler.
Even then, it wasn't truly flat so needed the high-build which, after flatting, has given me a reasonably smooth car ready for topcoat. The first bit was sanded three weeks ago and the last bit sanded this evening.
Obviously I have over-run on the 5 hour deadline for over coating!