All fibreglass panels that have been produced from a mould have a coloured surface, which is normally, white or grey, but they can come in any imaginable colour. That coloured surface is called a gel coat & is painted into the mould prior to the glass fibre matting being applied. The aim of the gel coat is to give a smooth, durable exterior surface to the fibreglass matt. The gel coat can be painted over, but it's a little bit more involved than painting a steel panel.
When a mould is being prepped for making a panel, it is first cleaned & then a number of coats of release wax are applied to allow the panel to "pop"& release from the mould easily. The gel coat is then either brushed or sprayed on, followed by a number of layers of matt. After curing, the panel is popped from the mould & trimmed. There will be a certain amount of release wax still impreganted on the surface of the gel coat, so it's important to make sure that this is completely removed otherwise it can affect the paint process at a later stage.
Use a wax/grease remover available from any automotive paint supplier & degrease the panel a good few times, using a fresh bit of rag each time to remove the degreaser. (if you use the same rag a number of times it becomes impregnated with the release wax & you end up just spreading the wax around.) I'd degrease it 4 or 5 times to be safe. Once it's been degreased, check over the surface for any signs of imperfections. The biggest imperfection that you come across with any fibreglass products are called "voids". This is where air becomes trapped between the glass matt & the gel coat if it's not been rollered out properly during the manufacturing stage. By heating the panel with a hot air gun or hairdryer (without frying the thing
) you will see either little lumps appear or small shallows in the gel coat surface. If you spike them with a sharp object like a Stanley blade you'll see that you can make a small indent in the surface under the pressure of the sharp object.If you do have any voids they need to be ground out & filled with body filler or stopper.
Once happy that the surface is devoid of voids or defects it's time to sand it down. I tend to start with a 180 grit paper (or disc on a DA sander) followed by a 320 grade paper. Don't use anything finer or the paint process won't adhere to the gel coat. At this point there shouldn't be any shiny spots on the surface anywhere. If there is, then re-sand those areas until there's no shine.Once it's flatted, de-grease again.
Ordinary primers won't adhere to the gel coat so you need to use an etch primer (most are acid based which causes the etch primer to "eat" into the surface & adhere to it.) if you're using rattle cans then use something like U-Pol acid 8. Alternatively you can use an Epoxy primer, which again will adhere to the surface very well. Both of these products will need to be coated with a high build primer once dry.You only need to give the panel 1 full coat of etch or Epoxy, to ensure the panel is fully covered. Too much etch/Epoxy & you'll get adhesion problems between coats. From this point on the painting process is the same as for any metal panel.Spray on 3 or 4 coats of high build primer & allow it to dry. Then spray the primer lightly with a dark colour (guide coat) this should be a mist over the whole of the primered panel. The aim of the guide coat is to enable you to see what's been flatted & what hasn't.
Sand the high build primer down with 320 or 400 grit wet & dry, followed by 600 grade. As you sand the primer down, the guide coat gets rubbed off first, if there's any guide coat left on the panel then it needs rubbing more until the guide coat's totally removed. You then know that it has had a uniform sanding. Once you're happy with the sanding, dry the panel off completely (don't allow moisture to sit on the surface for any great length of time as moisture can ingress into the primer,which is porous & cause problems later on with the top coat.) on flat panels I'd recommend using a rubber block (available from your paint supplier). The block will stop you getting tram lines in the primer caused by your fingers rubbing the primer unevenly. Once it's all prepped, degrease it all again & apply the top coats of colour as per normal.
Obviously the above is abbreviated & doesn't go into any one aspect in great detail, but if you have further questions on this, just ask I'll go into more detail on that aspect. Hope this helps, cheers.........Nige
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