On Wednesday, November 7, 2012 6:18:29 AM UTC-7, RL wrote:
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
Bob, Thanks for the info. Some of the Simtec products can be cut with a 50/50 mix of Acetone and Lacquer Thinner (Per the website). Also, for anyone using Prestec, help is just a phone call away. Same thing with many of the Eastwood products.