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MG MGB Technical - painting on top of existing paint

Now the weather has picked up I am aabout to do the bonnet (ally) of my roadster. I have stripped the top but only scuffed up the top coat and primed any bare ally on the underside. Stripping the paint here would be really tedious. Do I put top coat straight onto the existing top coat (sanded back) or do I primer onto the old top coat before putting the new top coat on? The rest of the car will be taken back to bare metal and etch primed. Apart from the few areas that I have had to use filler in (like where i dropped the door on the floor!!). I discovered that etch primer doesn't like plastic padding.
Steve Church

I normally sand the old paint, clean it really well with prep solvent and spray primer then paint.
Bruce Cunha

Sometimes the new paint compound may not be compatible with the old existing paint. This may cause an adverse reaction. IE: the new application may not adhere to the old or may not lay smooth especially if it's a different color or lacquer on enamel. That being said you may wan to prime the entire car to avoid these issues



79 mgb
gary hansen

To avoid any potential problems with non compatable paint types such as blisters, cracks, flaking etc you need to spray a coat of isolator first, then prime, then new top coat.
G Britnell

How many coats does it have already? When I restored my roadster and cut back the paint in many areas to treat rust I found some panels had been painted up to four times, with primer and top coats, that's about a dozen coats. The painter said he could feather it all in, but warned that the underlying layers could crack into islands. Each coat of paints shrinks over time, applying more and more tension to underlying layers, and eventually they give way. As he said he could take it back to bare metal for the same price that is what I chose. Mind you, he regretted that, it took him much longer to strip than he imagined.

I also had a Scimitar GTE that needed a small repair to a wheel arch, and they ended up having to respray the whole of the car forward of the windscreen due to reactions as mentioned.

Even though it's only the underneath I'd primer it, perhaps a small area first to check for reaction.
Paul Hunt

I've removed all the flakey bits of paint and filled over the bare metal to the level of the original paint so no feathering to do. I've sprayed some rattle can primer over all such areas. No adverse reaction so far. The remaining 'good, sound' paint has been rubbed down. I reckon I can spray some primer from the gun at the weekend. Just one coat to see if there is any reaction. Assuming the temperature is high enough. The top of the bonnet is stripped back to bare metal but I'm still profiling the front of the bonnet where it had sustained some dents. This is the original bonnet and started out as Teal blue. The PO had it painted BRG fater that straight onto the original top coat (I'm not sure if there was any primer between as the BRG had started to flake away leaving the original blue colour showing. I stripped the top of the bonnet with chemical stripper which was quite successfull.
Steve Church

It might not react right away. When i restored my BT GT it took several months before pin size blisters appeared on parts of the car that were not taken back to bare metal. By then the car was back in use & nothing could be done - i was gutted. Use a coat of isolator, the extra cost isn't much for peace of mind that your lovely new paintwork will stay that way.
G Britnell

Steve - I painted a fully restored mini clubman a lovely shade of Ducati yellow 2 years ago, after fastidious prepping of the original red topcoat. We flatted back to original primer,then primer-filler,lots of final flatting, then 3 topcoats of 2-pack yellow. For some reason,after 12 MONTHS the roof reacted in dull, flat patches. We had to re-sand the roof to original primer again, and re-finish.This time, we applied an isolator coat, and hopefully there will be no further reaction. Lesson : in my humble opinion, sand back to original primer, then definitely apply isolator , then prime and paint as required. Or of course go back to bare metal. Cheers ... John.
J.P. Hall

This thread was discussed between 06/04/2014 and 11/04/2014

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