Prep for painting - body filler?

[gazzamuffin]

Hi,

So I’m starting to tackle the body on my s3 elan.

All the usual paint cracks, but also paint on the front and rear lower half lifting and falling off.

Questions.
1) do I need to take all the paint off right down to the gel coat or can I flat it.

2) What filler do you recommend and would you cover the whole body or just where needed?

[Gordon Sauer]

My sense is, that if you have paint coming off, then you’ve got adhesion issues and got all come off. I’ve done chem stripping. I’ve done chipping away with a chisel and I’ve done sanding on three different cars and it all worked out OK then I repaired big areas because they had a bunch of cracks like the sidewall of the Elan s3 trunk when things move around in there, I fiberglass over with thin cloth and then I regel coat like it was originally and then spray on top of that with a catalyzed primer/filler and finally urethane single stage paint, Trying to avoid body putty whenever I could, but using it some to be sure surfaces were flat once I was in the primer filler stage. Primer filler goes on several times and then block sanding, air file sanding back til all smooth and end up with 600 and then three coats of color with some color, sanding finally, Gordon

[Andy8421]

If you don't strip, then the thinners used in paint spraying reacts with the layers of paint under the surface and can lead to witness shrinkage marks around previous repairs, edges of paint layers and so on.

Most of our cars are like a paint and filler 'club sandwich' under the skin, and the chances of getting a good, lasting finish aren't great if you don't go back to the gel coat.

The cars came out of the factory with blemishes and joins filled with a polyester resin based filler, so that seems a safe bet to use, though there are some refinishing firms that only use polyester resin and matting as a filler. Depending on how bad the surface is, some firms will sheath the whole car in tissue mat and resin prior to spraying.

The bible on repairs is the Miles Wilkins book, now long out of print. It comes up on eBay from time to time. There is a copy on Amazon, but the price is high.

https://www.amazon.co.uk/Restore-Fibreglass-Bodywork-Osprey-Restoration/dp/0850455561

[UAB807F]

1) do I need to take all the paint off right down to the gel coat or can I flat it.

Given the price of paint these days it makes no sense to leave anything behind on the car which could possibly react with the new paint. If you're paying someone else to do the painting then it's even more critical to ensure it's as clean as you can. The last thing you want is sinkage or rings appearing around repairs six months down the line.

Getting down to the bare fibreglass isn't as hard as you might think and you'll know that you've done the very best prep. you can.

Personally I have used paint stripper in the past even though it's not recommended on fibreglass cars. The trick is to watch it working and only use if to remove the upper paint layers. If it contacts bare fibreglass then some formulations will attack the gel coat and definitely make a mess of any filler repairs.

Once it's down to the original primer coats I use 80grit production paper to bare fibreglass and keep it dry.

2) What filler do you recommend and would you cover the whole body or just where needed?

I don't use the body fillers for any repairs, I use resin/surface tissue with chopped strand matting if needed to bridge gaps, etc. I do have a tin of Isopon P38 on the shelf which is probably 20yrs old now and I will use it but only to fill pinholes that come out when flatting a resin/surface tissue repair. I'll use 80/120grit production paper to get the profiles and no water at this stage.

I do use spray filler over the complete body, "U-Pol Reface" in my case, but there are others available. Again any sanding to remove nibs, dead flies, etc, is also dry, usually 240grit paper, maybe 320 before throwing on some primer.

I'll keep things dry until at least a couple of coats of primer are on, then use 240/320 wet. Alternating colours helps to stop you cutting back and wetting the U-Pol layer.

[steve lyle]

1) You need to take off any paint that isn't adhering. How far down? Until you get rid of the poor adhesion area/layer. That might not be to gelcoat, you may get to a previous paint layer or primer layer, find good adhesion, and stop.

2) Just where needed. But I got to ask - why would you use filler where it's not needed?

It sounds like my experience is different than some others - but when I resprayed my car, the existing paint had good adhesion, and was a urethane paint (i.e, catalyzed). So it didn't react at all to thinner - i.e., would be safe to paint or prime over.. I knew that because I tested it. It was also only one layer deep - again known because I checked. I say that because the paint consultants I checked with recommended stripping if there were already 2 or 3 layers of topcoat on the car. When I painted my MGB, it had at least 3 layers, each with primer, and I stripped it down to metal. I believe their issue with multiple layers has more to do with losing the panel feature lines - increasing radiuses, than with adhesion or compatibility. Urethane or polyester primers dry so quickly, there's not much time for them to react to the substrate beneath them. They seal very well.

Given the above, I flattened the existing paint with 200 grit, sprayed two coats of a urethane primer, flattened that, wet sanded to 400, and sprayed the color. All that was done after repairing various cracks, crazing, etc.. Those repairs were done with epoxy resin and various weights of cloth, generally topped with decking cloth (a very fine weave of fiberglass that didn't exist when Miles wrote his book, but basically the equivalent of tissue, only stronger). Then a final skim layer of polyester filler.

It's been over a year since the repairs were made, and maybe 9 months since the car was painted, and I can't find any sign of the repairs coming through. That's probably somewhat due to the fact that most of each repair consisted of epoxy resin, and that doesn't shrink anywhere near as much over time as polyester does. I used a bare minimum of polyester filler.

[gazzamuffin]

Thanks everyone, all the information is helpful.

So where the paint is lifting I have started to take it right back and finding that the car has been a couple of colours in its life.

On closer inspection of other areas of the body I can see what looks like tiny bubbles under the paint could this be the last paint job reacting with the coats underneath.

I will take some pics and add them shortly

[StressCraxx]

From my humble experience, the tiny bubbles are from moisture wicking into the glass fibers at one time. With some heat cycles, the moisture expands and causes micro blisters. I used gentle heating with an infrared or convection heater at about 40C helped bake the moisture out. Then I used a fiberglass sealer coat and primed for flatting with block and sandpaper.

[Andy8421]

Microblisters can have many causes, but the main challenge is that polyester resin isn't completely stable, and over time will react with moisture. Boat owners have it much worse than we do, have a look here:

https://www.makewoodgood.co.uk/causes-of-osmosis-blistering-grp/

The usual treatment is to try to dry the body as much as possible before spraying. An interesting point Option 1 (body refinishers) made to me was that while everyone focuses on the top surface, water can enter the grp from behind during its life, so it is important to seal the back surface of the grp, particularly in areas subject to road spray (front wheel arches in particular).

[UAB807F]

Thanks everyone, all the information is helpful.

So where the paint is lifting I have started to take it right back and finding that the car has been a couple of colours in its life.

On closer inspection of other areas of the body I can see what looks like tiny bubbles under the paint could this be the last paint job reacting with the coats underneath.

I will take some pics and add them shortly

Pics would help the diagnosis but on the basis of what you've written then I would be taking all the paint off. If you have more than one colour on the car then although the top layer might be safe to spray over and not react with new paint, the previous layers are an unknown.

My reasoning is that at somewhere on the car you'll end up repairing gelcoat cracks or previous damage and although you can feather out the repair and previous layers of paint to make a smooth transition to the "top" layer, there's every chance that at some point in the future you will get paint sinkage in these areas.

Having been there I'd put primer on & get some reaction from the previous paint rings, leave it overnight, flat it smooth, re-prime and it would look ok. Top coats on and.... weeks/months later it comes back. Hence I always go to bare shells now.

The bubbles/blisters are usually caused by moisture in the paint layers. It's a mystery how this happens to me, I know there are new Emiras only months old with paint blisters and it seems to be a problem with modern Lotus cars. You'd think that a modern factory finish wouldn't have such things but it seems so. AFAIK the approved procedure is to strip the paint off completely and start again, something that would really annoy me if I'd paid £70-£80k for a new toy.

But in your case I'm like a stuck record with "take it all off". We don't know if the blisters are coming from glass fibres/poor filler repairs/bad preparation or even previous paint and the last thing you'll want is them coming back in your new paint.

Rather annoyingly this winter I found a couple on my Elise which at some point I'll have to fix.

[UAB807F]

An interesting point Option 1 (body refinishers) made to me was that while everyone focuses on the top surface, water can enter the grp from behind during its life, so it is important to seal the back surface of the grp, particularly in areas subject to road spray (front wheel arches in particular).

Good link Andy and a great point.

This winter I found some blisters at the rear clam on my Elise which I've owned from new and is still in the original paint. 25yrs old and for 24yrs no problems at all but somehow, this winter, a few blisters have appeared low down on the rear arches.

No upper paint damage, never been painted and yet they've come out. I can only put it down to moisture wicking through the relatively thin grp of the Elise clamshells (they are thin)