Re: Gel Coat Spraying Equipment

[Dan Marotta]

I've sprayed gelcoat with an airbrush. It's been a long time so I don't
remember for sure, but I may have thinned it a bit with acetone.

It worked well.


"Terry Edmonds" <terry-...@uiowa.edu> wrote in message
news:edbb2d2a-2a37-41c5...@googlegroups.com...
>I am repairing a small scratch in gelcoat and looking for recommendations
>on a suitable sprayer for a small job. Wondering if liquid gelcoat is to
>viscous to work in an airbrush?

[POPS]

Dan Marotta;827871 Wrote:
> I've sprayed gelcoat with an airbrush. It's been a long time so I don't
>
> remember for sure, but I may have thinned it a bit with acetone.
>
> It worked well.
>
>
> "Terry Edmonds" terry-...@uiowa.edu wrote in message

> news:edbb2d2a-2a37-41c5...@googlegroups.com...-

> I am repairing a small scratch in gelcoat and looking for
> recommendations
> on a suitable sprayer for a small job. Wondering if liquid gelcoat is to
>

> viscous to work in an airbrush? -

Yep, Cut the gel coat with acetone until you get the spray you like.
Test spray on smooth vertical surface - no runs. Use a good, good bit
more catalyst then you think. Get the scratch onto the horizontal if
possible. If not, wait 10 minutes or so between light coats - no sags...
no over spray outside your tape off... Clean the scratch super well
with acetone. Tape the area off pretty close to the scratch and out
another 10 inches with newspaper. Should the "scratch" be filled? Almost
sounds like it...scratches usually can be taken out with wet 600 -1000 -
polish... Over spray the gel coat with PVA so the gelcoat is cut off
from the air and cures hard because you will need to sand with wet 280
400 600 1000 and polish; if that's where you're going. Same procedure
for larger areas too.

PVA http://tinyurl.com/cy6mmkq

These sprayers work real well too.
http://shop.preval.com/collections/frontpage/products/preval-val-pak

Gelcoat never lays out flat like a topcoat spray job. Sanding and
polishing always required..

LOL




--
POPS

[Brad]

On Nov 2, 6:31 pm, "Dan Marotta" <dcmaro...@earthlink.net> wrote:
> I've sprayed gelcoat with an airbrush.  It's been a long time so I don't
> remember for sure, but I may have thinned it a bit with acetone.
>
> It worked well.
>

> "Terry Edmonds" <terry-edmo...@uiowa.edu> wrote in message

>
> news:edbb2d2a-2a37-41c5...@googlegroups.com...
>
>
>
>
>
>
>
> >I am repairing a small scratch in gelcoat and looking for recommendations
> >on a suitable sprayer for a small job. Wondering if liquid gelcoat is to
> >viscous to work in an airbrush?

I've had good success with a Harbor Fraud HVLP conversion gun, the tip
is 1.4mm....IIRC.

I use Prestec 2781 and thin with slow lacquer thinner, used to use
acetone but last paint job was thin with lacquer thinner with great
results.

Never done spot in paint repair before, just whole
surface.............let me know how your repair paint comes out.

Brad

[kjones]

POPS;827901 Wrote:
> Yep, Cut the gel coat with acetone until you get the spray you like.
> Test spray on smooth vertical surface - no runs. Use a good, good bit
> more catalyst then you think. Get the scratch onto the horizontal if
> possible. If not, wait 10 minutes or so between light coats - no sags...
> no over spray outside your tape off...
>

> Just a note that using acetone was mentioned to reduce gelcoat...most
> manufacturers now recommend styrene or even better MEK as thinner. If
> doing a large area pros will use patch aid
> (http://www.bottompaintstore.com/patch-aid-for-gel-coat-p-12410.html) or
> similar products (both the wax and thinner).It is believed that acetone
> has a tendency to yellow more over time and tend to develop cracks.




--
kjones

[RL]

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

[Brad]

Great info Bob.....thanks!

Brad

[K]

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.

[stephen...@sympatico.ca]

A lot of info on spraying ... but ... if you are just repairing a "small scratch" then just brush it on and start sanding/polishing!!!

[stephen...@sympatico.ca]

On Wednesday, November 7, 2012 8:18:29 AM UTC-5, 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.
>
>

I've seen others make this distinction whenever this topic comes up. BUT gel coats can also be applied as topcoats. The MGS (Scheufler) website has a nice document that explains the different methods for applying T35 in mold or as a topcoat. When applied as a topcoat "to repair small areas, the following procedure can be used: Add 10% Thinner HF to the gelcoat, mix thoroughly with 10% Hardener SF. Add an additional 10 - 30% acetone." The instructions go on to describe the spray technique which involves thin coats, leaving enough time for the solvents to "disappear" (5 to 10 minutes) followed by additional coats likewise laid wet on wet.