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General Matra related => General Discussion => Topic started by: andyowl on June 10, 2009, 08:47:45 am

Title: Paint blisters
Post by: andyowl on June 10, 2009, 08:47:45 am
I did a search for this topic and did not find anything! Forgive me if it has been discussed before!

We have found that our current Bagheera project car has many small blisters in the surface of the GRP when all the paint has been taken off. It was stored for 15 years outside with only a car cover over it. There were at least four coats of paint on the surface which had cracked, flaked and looked terrible. It had to go. It was hard work but we got it back to the basic GRP. Then we found the blisters.

Now we want to paint it but even our primer paint has not covered all the craters where the blisters have burst. We are rubbing down the primer but the blisters are still there. Typically about 2-3mm diameter and deep enough to feel with your finger tips.

I assumne the blisters are in the "Gel Coat" and I don't want to rub this away or we shall be through to the raw fibres.

Any advice?

Andy Owler

Title: Re: Paint blisters
Post by: Fred on June 10, 2009, 11:28:57 am
It's called osmosis and Alpine-Renaults of much younger vintage are also sufferers. >:(   A lot of work is involved.  My Bagheera has some evidence of it.  I'm heading for England in a day or so to pick up some spare panels so my fingers are crossed that they will be free of it.  ;)

Some information here from Spyros' site:

Osmosis - blistering, osmosis, wicking and hydrolysis

GOOGLE BOOKS - The Restoration Handbook

AMAZON UK - The Glassfibre Handbook

The Glassfibre Handbook

Title: Re: Paint blisters
Post by: andyowl on June 10, 2009, 12:16:32 pm
Very helpful! Particularly the article: "Osmosis - blistering, osmosis, wicking and hydrolysis" about GRP yachts. I particularly liked the quote about many yachts on the world's oceans cruising about quite happily with underwater hulls looking like bubble-wrap!

So we will do our best and apply the paint and admire it when the car is doing 45km/hr and you cannot see the minor imperfections. Known as a "30MPH paint job".

Saved me hours of work. Thanks again.


Title: Re: Paint blisters
Post by: JL on June 10, 2009, 03:24:04 pm
I have similar problems.,1440.0.html


Title: Re: Paint blisters
Post by: andyowl on June 10, 2009, 03:55:36 pm
I see that your report is only three weeks ago. What has happened since then??

Andy Owler

Title: Re: Paint blisters
Post by: JL on June 10, 2009, 10:26:53 pm
I am still getting the bodywork back to the GRP layer. I beleive the problem stems from the fact that the original factory laquer let water under the paint and started the blisters. My car has then had a further 3 paint layers to try and rectify the problem but I believe the original layer was never allowed to dry off and the subsequent layers trapped the water in just continuing the blistering. When I first took the 4 paint layers back to primer, where the blisters were a damp patch appeared in the dust; my car has now been in the dry for at least 8 weeks and I have recently started to sand off more of the primer layer and these damp patches have not reappeared leading me to believe that the car is now dry. When I have finally got the body back to the base layer I will give it some coats of epoxy primer to seal the body, the epoxy primer being waterproof. I will then paint the car with a good quality 2 pack paint and laquer. As a belt and braces I will seal the underside of the body where I can gain access with Gravitex which is also a sealer and waterproofer - that is the theory anyway . I just what I  doing am works as it has been a bit of a mission sanding everything down.


Title: Re: Paint blisters
Post by: andyowl on June 11, 2009, 12:09:11 am
I have got bored with rubbing down and have decided to paint it anyway. The yacht article on osmosis suggests that one should accept that osmosis will always arrive. The only unknown is when. Paint is not a once-and-for-all treatment.

On Grey Baggy, which was painted in the shop of our control panel business, with the full treatment 2 pack epoxy paint base coat with cellulose top coats, is still free of blisters 17 years later. I lost track of the man-hours we spent but the job was superb and has lasted very well. On a competition car, is it worth the effort? I'm not convinced. I may feel differently when we have done the job.

Conclusion:- Don't keep your car under a "weatherproof" cover. Keep it dry in a garage if you can or keep it under a shelter if you have no garage, or just keep it in the open. But don't cover it and let the humidity get trapped underneath.

Thanks for all the contributions. Very helpful.

Andy Owler