Could someone please tell me whether it is necessary to add a surfacing agent (like styrene and paraffin wax) to an epoxy hot coat? Or is epoxy simply sandable once it dries without any surfacing agent?
Epoxy cured tack free so no need surfacing agent. But epoxy can react with air humidity, a grease film appear at surface, clogging sand paper. Surfacing agent can help to avoid this. Most modern epoxy used in dry climat don’t react anymore. Because those epoxy are voc free they lack surface tensil so they are really sensible to any contaminent on covered surface so add some solvent in hotcoat can help to reduce fish eyes and pin holes.
Something that’s going to save you a ton of headache on epoxy hotcoats is the brush you use. The cheap brushes leave tons of brushmarks that you have to sand out. A Purde Nylox is the best. You can clean them if you are quick enough. A brush comb, and vinegar, then a brush spinner for the first washes. The first vinegar wash turns the epoxy into a white goo thatg can be combed out. Then a second washing with xylene. Spin it dry… But make sure all the epoxy is out. It usually takes a few washings to clean it. But a good brush will save you hours of sanding
To the original question: No it is not necessary
Quality epoxy surfboard resin that flows like water will not need surfacing agent. Those epoxy resins that are maple syrup thick either need to be warmed up ,if you are in a cold climate, or surfacing agent added to make less viscous for eaiser workabilty. I use Green room resin and have no longer used surfacing agents. All other epoxy resins I have xylene available to thin out if necessary. I use disposable 4" chip brushed tapped off to control stray brush hairs.
Attached photos are Filler coats of initial laminations. Green Room Resin No 7 with regular fast hardener. I am a vacuum bagger not a shper or glasser. Have used most if not all the available epoxy resins and Green room really makes up for my lack of abilities in the glassing area. Chip brushes that I throw away. This resin levels out and sands easily as in 1-2 abrasive pads per board.Thje other resins are fine for vac bagging but Green Room does it all.
BTW… Blue and white board done while it was raining outside. Humidity concerns are no longer an issue with Green Room resin. Never noticed a problem with humidity with othert resins but it always was an available excuse for my lack of abilities to blame on.
LAST PHOTO is what initial lamination looks like prior to filler coat of epoxy.
hope this helps.
