I have not painted a whole board this way, only did repairs, but the easiest is just to lightly paint the board & sand it very gingerly.

I like the "stripes" effect personnally: try to make color/carbon stripes on purpose:



Or the 2nd from the left:

If I'm understanding you correctly you are proposing using micro balloons to increase hardness the resin...micro balloons do the opposite...they soften up the surface, that's why it sands easier than straight resin...if you wanted to make the surface harder you would add cabosil. Cabosil, however is much harder to sand, adds considerably more weight per volume, and adds the nightmare of adjoining surfaces being different degrees of hardness. It is much harder to feather a cabosil/resin coat into a raw glass/resin surface, the sander will just sit there on the cabosil batch cresting a lot of heat without cutting much while adjoining laminated surfaces get sanded down at a much faster rate. If you have a properly mixed epoxy and add cabosil to it is somewhere between the effort of sanding raw glass laminate and sanding cured concrete...not fun...the resin/laminate by itself with resin filler coat is as hard as you need....as they say "don't ask me how I know"

You can try to wet sand this way, finbox test is included as wel

rghdc said..
Colas...those red/black striping effects is what I was envisioning by putting a rather dry thin clear coat with heavy brush marks pulled in it just before it kicks to add like a grain texture...then go over the cured texture without sanding with a color coat...when the surface sanded flat the high grain ridges in the clear would show the underlying carbon and the lower areas would retain more of the color coat.

I'd advise you to just experiment: take some black material (painted wood? Black plastic sheet?), and put your ideas to the test.

Some of the ideas I had did not work well in practice, but removing fresh paint with a Solvent F soaked cloth seemed to work too. But sanding was the most easy and the most reliable method.

Not that the more you see the carbon, the less your board will like to be in the sun...

rghdc, I disagree with your choice of word: miscrosphere gives one of the hardest composite mix, which makes it so easy to sand by the combination hardness + easily abrased. What you describe of carbosil is strength or resistance, which I agree is harder to sand.
pure epoxy resin does not sand well as it stays a bit flexible (which makes less prone to cracks than polyester resin) and becomes a paste with the heat of the sander.

Colas & Kami I appreciate your input.
My reasoning is based on an advanced composite seminar put on by a engineering PHD from FIT... his arguments still seem solid today.

Our varing perspectives may revolve around the expected useful life expectancy of an object and the consequence of failure. No one wants a fragil board that is prone to damage or failure, but there is quite a huge difference from an aircraft that experiences the same level of damage in flight.

I may have to consider the unthinkable and add micro balloons to laminating resin. In the past all parties setting composit standards I have worked with have been adamantly opposed to such, as their testing showed greater chance of inter laminar sheer and chance of delamination between layers. In a board it may be worth the trade off as the core has negligible bonding strength anyway.

Colas I have not had any problem sanding Resin Research brand epoxy...been using it since the at least 1990 if not earlier. Some of the earlier versions of epoxy I used in custom boat building fit your description, mainly Epon and Ebond brands...very thick oily with a terrible sliminess blush...that stuff was so nasty and a bear to sand. West system & system 3 are somewhere in the middle with blush and sand-ability...RR is a dream compared to the others iv used....so if any of those brands are your benchmark I could see where you would think sanding epoxy has to be a nightmare.

I'd say the RR resin I use is very much like sanding a quality surfboard grade polyester...except you don't have to deal with the repercussions of the wax/styrene additive. Didn't mean to put words in your mouth, so to speak, by using the term "nightmare" but the Epon and Ebond epoxies I were referring to are a nightmare to sand.
As far as heat build up when sanding, some of that is based on the sander, sander disk rpm, grit and type of paper, and type of back up disk your using. I use a 6" Port-a-cable duel action sander with a medium flexible disk. It runs extreamly flat without bounce or any other bad habits.
Looking at the Starboard repair video... Wouldn't let him close to a board with a hard backing disk like that...and those sandpaper grits must be in metric units, because you would not even think about sanding down that repair with 20 grit..,in the US that almost like having small stones glue to the paper. You should never put three layers on of the same size as you create a hard point in either side of the repair...and filler should be applied to minimize sanding. The cloth should have been staggered and he could have covered and taped a light weight piece of plastic tightly over it and hardly needed any filler and minimal sanding. A backyard repair would end up horribly following that video ...when you use those tools 8-10 hrs a day for years you can get away with all kinds of crap...an amateur would end up with a far bigger problem than a small crack following that method.
Sorry if that sounds harsh, and yes he gets it done, but thats not the proper way to go about it. I have been responsible for complete shop oversight of building multimillion dollar yachts in the past and that is poor execution. Makes one wonder about the quality under those "slick" paint jobs from the production builders. With custom guy you see what you get...for the most part...not so much with the painted boards.

I agee with RGDHC, I would never ever use these tools on a repair like this and the same with the glassing.