Before you start painting gelcoat check the following.
Most gelcoats you paint will have releasing agents (wax)on the surface.
this substance is left over from the manufacturing process the (mould it was made from) the reason it is there is that the mould can be released from the object you are about to paint.
The quality of fiberglass
resin, matting and mould used, will have an effect on what you need to do to paint it (how much work you will need to do).
The first step to do is decide if what you are about to paint say like panels of a car, bike fairing, truck bumper, custom made car, if they actually fit! if they don’t , you will need to trim them down to size this should be done before any painting is done.
degrease the fiberglass /gelcoated
do this with panel wipe, white spirits, acetone, wipe down the area and before the panel wipe has evaporated wipe it clean and dry with a lint free cloth. (never sand the surface at the start)
It is important not to abrasive the surface at this stage because we don’t want any mould releasing agents driven further into the gelcoat because this will delay the whole painting process and lead to a poor paint finish.
You can test to see if you have removed all the releasing agents by pouring water over the object if the water beads up there is still releasing agent on the surface so you will need to keep repeating the process until the water runs flat.
The gelcoat is normally made with either epoxy or polyurethane resins, however the sanding process is the same, the surface will be slick and hard sometimes giving a very glossy surface. as a rule of thumb when painting you cannot spray paint over a hard glossy surface, the paint will delaminate (peel) in a very short time, there are exceptions to this rule, you can use 2k direct adhesion paints.
Start the sanding process with P180 grit sandpaper make sure when you have finished sanding that the surface is matt in appearance there should be absolutely no glossy slick parts left anywhere, for difficult to reach parts you can use a green scuff pad.
Now panel wipe the area same as before wipe on wipe off with a dry lint free cloth don’t use coloured cloths like old red cotton t-shirts because dye from the fabric can sometimes run (bleeding) a lot of the dyes they use can also be silicon based! a double problem which is a bit of a disaster considering the effort you will be putting into removing releasing agents.
I recommend you use a 2k acrylic hi-build primer (auto-quality), mix up the primer using the correct mixing ratios and spray with a proper primer gun. Here is a link to an excellent Auto Primer and correct spray gun option cheaper gun.
Spray two to three coats allowing 10-15 minutes between coats do not sand between coats, let the primer dry overnight if possible you can speed up the drying process with infrared lamps or placing the item in a low bake oven. But you can run into problems with trapped air in the fiberglass popping out or warping of the object you are trying to paint.
You now need to guide coat the surface
if you’re not sure what guide coating is?
all it means is that you get a can of aerosol black and spray a mist coat over the primed area let it dry should take 15 minutes
- then you start sanding you will notice high and low spots on the panels so you will need to keep sanding and in some cases you may need to repeat the guide coating and maybe the priming process if the panels are very bad.
- Use P600 grit sanding paper or if you have an orbital sander you can use P400 grit discs.
Now when you have finished with the priming
Blow of all the sanding dust with a blowgun, panel wipe the surface wipe on wipe off.
Now its time for the spray painting process
I would stick with two pack paints all the way they are chemical cure as opposed to air dry (solvent evaporation) so will last alot longer and will protect the fiberglass a lot better.
- stick to the manufacturer’s recommendations apply two to three coats 10-15 minutes between coats don’t try and paint it in one go, you will end up with a heavy textured paint finish and probably lots of runs especially on the hard edges.
leave the paint to dry for about twenty four hours at this stage you can flat down the painted finish if you need to.
- with P1500 paper to remove any dirt and then P2000 grit paper
- next onto rubbing compound then polish.