About to make my first Outdoor MDF sign...

[billp]

Just a historical note; back in the '70s when polyester was still the glop du jour (before the epoxies, and S glass, and vinylesters, etc.), the entire marine industry sprayed,troweled, and hand rolled polyester resin and cloth into, and on to just about every production boat on the market. Then a few years later they learned about 'shortcuts' and started to see just how little resin to cloth they could get away with, ( and they were on the right track since a lower resin to cloth ratio is actually a stonger bond). But in many cases "R & D projects" were sold to consumers who raised their sails, and left the dock.
THEN a few years later during haul outs people would notice little 'blisters' protruding through the bottom paint of their hulls. It almost looked like pizza. Apparently 'osmotic blistering' was
now becoming an epidemic and people were getting LEAKS in their supposedly impregnable hulls.
Turns out that the only sure cure for the issue was to go in and physically dig out each individual blister, (making sure that you got ALL of it out), and then filling in the resultant cavity with- epoxy.
Wooden Boat magazine had a GREAT time with all of this (after all polyester was what pretty much shut down many traditional boatbuilding shops), and dubbed the offending blisters as being sure signs of a major infestation of the "polyestermites". They even did a tongue in cheek article describing the dangers of this creature, and they did it SO well that before long many boatyards were being overwhelmed with boat owners who wanted to have said "bugs" removed from their hull. Many insurance companies wound up paying for entire bottoms to be ground off and completely resurfaced, with -epoxy.
While each product has it's own characteristics from an artisan's viewpoint I have to say that epoxies are much easier to be around as they don't produce the "Bondo smell" which polyesters permeate (and that lingers in your shop for a long time...). As PeterJ points out above epoxies are thermoreactive meaning that you can control curing time by controlling the temperature of the space you are working in. Polyesters rely on differing proportions of the resin, and MEK which is the primary catalyst. A few drops more/less of MEK and your project can cure in either minutes, or months...AND a polyester skin can glaze/crack as it doesn't have anywhere near the inherent strength of epoxy.
Joe is certainly on target when he points out that we rarely get much call for submerged signage these days. But if I want to finish up a project, and not get a call back, I'll go with the epoxy every time.

[jim_hansen]

Bill it was the rising cost of iso-polyester resin in the oil embargo years that drove boat builders to use the less noble ortho-polyester resins (cheaper) that caused the blistering problems as well as application methods (external mix chopper guns)and I've seen too much of that. There is nothing wrong with a thin laminate when done right. But for coating wood for boats or signs or anything else you can't beat epoxy. I use lots of it and I get it from Jamestown Distributors.com

[gerald_d]

I also understand that "osmosis" had a lot to do with the glass-fibers being poorly covered/sealed. Sometimes the tips of the fibers protruded through the resin and acted as a wick to draw the moisture in.

Anyway, the polyester vs. epoxy debate is well covered on many fora. Generally acknowledged that epoxy is better than polyester, but does it warrant the extra price for the application? It is like comparing silver and gold.

[elcruisr]

Actually while working as a marine surveyor I studied the whole blister problem very thouroughly. The causes are many, not one, and we don't want to get into them here. We don't have room! Even iso resin can blister. The only resins that have shown to be pretty much free of it is the vinylesters and epoxies. The reason seems to be because they are moisture barriers and polyesters are not. Worksmanship and application systems also factor in for any resin system. You could look at vinylesters as well but they are nearly the price of epoxies and have short shelf lives once "promoted" and ready to use.

Eric

[marshawk]

We get around all of this by not using MDF for signs.

Granted, I don't know all of your separate situations or what the jobs call for, but we have a lot of success using Spanish Cedar, cedar sign blanks and Signfoam for our products and a spar varnish as a top coat.

Extira works well, but kinda gets everywhere (including lungs).

Chip

[jay_p]

After reading here, and hearing Doc Polumbo sing the praises of Mas Epoxy, I finally have a student who is using it on the hull of his RC controlled craft. Does anyone have any tricks for getting a nice smooth application? What he has put on so far, has quite a few runs etc. I suggested that he sand it down smooth, and apply a topcoat, but I am not sure that it would be any smoother than what he has now.

Any ideas? We are over 600 miles from the ocean, so we don't get a lot of experience with these marine-type products.

Thanks in advance,

Jay

[billp]

Jay,
How did he apply the first coat? When the shape allows I roll it on with a tight foam roller, (one of those little white jobs you can get at a home improvement center..) If that isn't possible I use small "flux brushes" and really spread the coat on thin. Mix small amounts of epoxy when you do this as there is the issue of "mass" causing it to cure in the container faster.When this starts to happen the tendency is for people to speed up their coating process, and the quality of the job goes downhill fast.
Runs/sags are usually an indication that there is too much resin on the piece. If the surfaces to be coated are vertical it may help to just turn the piece on it's side.
Since there is already a first coat in place your suggestion of sanding it smooth is a good one, (120-150 grit will take the surface down fast unless it's really lumpy..) A major plus with MAS is that it does sand easily. Then try one of the above methods. A side benefit of sanding will be that it should leave a little "tooth"to the surface, and this may also help to keep the resin in place.
Multiple thin coats, are better protection than less, but heavier coats. A little sanding in between each coat will also improve the adhesion of the succeeding layers.
Now that I have said all of the above, I'm thinking that epoxy ( any epoxy) might be a little heavy for something that's going to fly (unless this is a good sized RC plane). I might be tempted to do multiple coats of shellac instead. The benefits here are;
very good sealing because the shellac is thinner than epoxy, and therefore should penetrate the pores more easily. Some people prefer a "spit coat" which is usually a mix of 3-5 parts alcohol to 1 part shellac. Being thinner this penetrates faster, and further into the surface.
The alcohol base of shellac causes each coat to dry much faster, and that lets you achieve more coats in the same amount of time.
Shellac can be left uncovered (whereas epoxy NEEDS a topcoat as it's only "enemy" is ultra violet light),OR top coated with just about ANY paint. If there is a need to have a clear finish an epoxy or shellac base then top coated with a good marine varnish having a UV filter in it is another proven method we used on wooden masts down in the Florida Keys for years.
And if this is still not enough protection you could THEN use epoxy on top of the shellac (which is pretty much the process I use in most applications anyway since shellac's properties which I have just listed make it one of the best prep coats I've ever used...)
I also just remembered that early in the history of aviation I believe they too were using shellac as a coating on everything from struts to wing coatings...

[jay_p]

Thanks a lot for the great info Bill. This is exactly the kind of stuff that I needed to know. The student is actually making a rc controlled sailboat in the 1 meter class. He knows more about the boat than I do, and I am as much a facilitator as anything.

Jay