FWIW, let me offer some information on gel coat. Beginning with
coatings terminology –

Paint – Dries when solvent vehicle evaporates. There are thousands of
formulations and variations.

Gel Coat – Cures during the molecular crosslinking (polymerization)
process. Technically known as “unsaturated polyester gel coat”, this
product is formulated as an in-mold coating for molded composites
parts. With a few exceptions, almost every sailplane has been produced
with in-mold gel coat, including those that are post-finished with a
polyurethane “paint” as a secondary coating.

Polyester Topcoat – Cures and dries. A hybrid consisting of
unsaturated polyester resin formulated with a solvent vehicle.
Products such as Prestec and Duratec are topcoats, not gel coat. Used
for re-finishing original in-mold gel coat surfaces.

Gel coat is not formulated to be thinned, except under very limited
circumstances. Thinning gel coat with acetone (or other solvents) can
result in permanent under-cure (incomplete polymerization) of the
product. Many general practitioners believe that the solvent “flashes
off” like a solvent vehicle in paint. However, what actually happens
is that the solvent combines with the styrene monomer in the
formulation, and all of the solvent does not evaporate during the
cross-linking process. The result is a low cross-linking density… i.e.
undercure. The ultimate result of undercure is poor weatherability
exhibited by color change, dullness or chalking. Thinning gel coat
with styrene is a prime cause of yellowing with UV exposure, as
styrene has poor UV resistance. There are some additives available
from gel coat manufacturers for patching, such as Patch-Aid and Speed-
Patch. These consist of the gel coat base-resin, a promoter, and a wax
additive for a tack-free surface cure, but even these additives can
only be used up to a level of about 15% by weight.

When gel coat is thinned enough to flow through something like an
airbrush or a Preval applicator it is not going to reach its desired
curing performance. The proper spray equipment to use with gel coat is
a top feed gravity gun, or a pressure pot spray gun. With the
appropriate equipment, gel coat can and should be sprayed without
thinning the material.

Another critical factor with polyester gel coat is that of spraying
the appropriate thickness. This material is formulated to be sprayed
at a thickness of between 14-24 mils (0.001”). Any wet film below
about 14 mils produces enough surface evaporation to deplete the
styrene crosslinking agent to the point where there is not enough
monomer left to fully polymerize. Again, the problem with undercure is
low gloss and poor weathering.

Polyester Topcoats (Prestec, Duratec. Etc.) are formulated for limited
thinning - Typically to around 15% of a specified solvent. To extract
the required performance from these products it is critical to use
only the solvent specified by the manufacturer and in no greater than
the specified range.

Temperature and initiator (catalyst) levels are also critical to a
proper cure. Generally a minimum temperature of 70 degrees F is
required for gel coat and topcoat. At lower temperatures the initial
polymerization is slow and does not become complete even over an
extended time. The material will appear and feel cured but the
crosslinking density will be low.

The initiator level (MEKP) is predicated on the weight of the gel coat
or topcoat, not the combined weight of the material and thinning
agent. There is a narrow range of acceptable initiator (catalyst)
level depending on the material – For gel coat that is generally
between 1.8% to 3% by weight, with polyester topcoats it’s generally
2% to 4% by weight. Note that all MEKP is not the same… there are many
formulations. It is important to use the initiator specified by the
manufacturer. Read the spec sheet and use the MEKP specified for the
lab tests.

To answer the original question – When spraying gel coat, a top feed
gravity gun with passages large enough to spray straight (non-thinned)
gel coat will produce maximum performance from the cured material.
Polyester topcoats are inherently lower in viscosity and can be
sprayed with smaller spray guns, keeping in mind that improper
thinning reduces the effectiveness of the product cure.

There are a lot of homegrown solutions and “we’ve always done it that
way” folk-knowledge in the composites repair business (sailplanes,
boats, etc.), but the technical facts are available to assist in
producing high quality work.

Bob