how to get a thick hotcoat??

There is any technique to get a thick hotcoat so i can sand it vrey smooth??My hotcoats have allways very lumps!

Thank you all

From Portuga…

~You could do two hotcoats (lightly sand between coats if using polyester).

~Set off the resin faster (would stop drainage - but also could make the lumps worse by not allowing it to level)

~Add microballoons (can come out blotchy or cloudy).

Do you have any pic’s of the problem. The hotcoat shouldn’t be that lumpy.



Depends what you mean by lumpy.

Most of my hotcoats came out looking like gloss coats, with a preponderence of dust specks to be sanded out.

If you want real depth, hot a layer of 4 or even 6oz cloth on.

RE: double hotcoating: No need to sand between coats, just do the second as soon as the first has gelled and cooled somewhat. The second will stick and sand fine as long as you don’t let it get too hard. Just hit the second while the resin is still somewhat rubbery.

A lumpy hotcoat is usually just following a lumpy lam.

Its not said (written) often enough, but to end up with a smooth board, EVERY layer must be sanded smooth before the next one goes on…if your shape is lumpy, the lam will be worse…if your lam is lumpy, the hotcoat will be worse…etc.

Sand that lam as if its your final product. If you sand through the cloth, it was a bad / high spot anyway and now’s the time to patch it. Once you have perfect, smooth, weave-showing glass coverage, that’s the time to hotcoat. If you’re trying to take out lumps by sanding the hotcoat, that’s too late.

Very interesting! I’ve had the same problem and gradually have been improving lam/hotcoat, etc. But now I have in the works a Balsa Fish (a’l Bert) that I just finished putting the first lam (5oz carbon) on (1 lb. EPS, 4oz. S glass horizontal stringer).

To get rid of the ‘lumps’ did the opposite…Instead of sanding down I mixed up a small (1 1/2oz.) batch of epoxy and put enough glass bubbles in it then squeegied it into all the low areas. The weight is negligable. 6’3" and so far (with out the balsa and a top layer of 4 oz. S glass… under 4 lbs!


A couple of things will make your hotcoat lumpier than grandma’s gravy at Thanksgiving.

  1. Old resin. Get fresher resin.

  2. Not mixing your own surfacing agent well enough (if you roll your own sanding resin).

  3. Not mixing the catalyst in well enough. Get a wire beater from the $.99 store and whip it in.

  4. Contamination. If you touch it with bare dirty hands or gloves, that is bad. Clean with acetone.

  5. Really hot and humid weather. Wait for the weather to cool down a bit.

This is not a complete list, but it is a start.

I agree with Benny 100%…Starts with your outline/shape and snowballs from there!

Hey Schroeder, good advice, welcome to the mix!

Let me add that hot coats are called hot coats cuz they’re supposed to go off fast, so the resin doesn’t drain, and a lot stays on the board for sanding.

I never sanded any lam and don’t expect to. All my smoothing happens with the Milwaukee, on the hot coat. I am thinking, though, of basting the occasional freelap, just maybe four or five inches wide. If I do, it will be lam resin only so there will be no adhesion problems. Any “second hotcoat” that you contemplate oughta be only lam resin. If using UV, it will cure enough in ten minutes to be overcoated with true hot coat resin (including surfacing agent).

To get a smooth hotcoat, start out with a smooth lam. That means a good squeegee technique. When the lam is hard, use a file/surform/40 grit/die grinder to smooth out any lumps and bumps after doing each side. Also using the correct width and thickness of brush for hotcoating will reduce the amount of brush lines that need to be sanded out. If you’re freelapping and there’s alot of strings and mess on the lam, it’s easier to sand those out in the hotcoat than on the lam. If too much weave is showing, put on a second hotcoat thinned with styrene or just use a glosscoat.

All hotcoats have some little bumpy imperfections, after all, it’s the coat you are doing to fill voids and sand smooth. File or sand off the lap lumps etc, but you shouldn’t need to sand the whole board.

A little cabosil in the hotcoat will thicken it up, add a little only. And then make sure you sand it before it goes rock hard.

I’m not a big fan of double hotcoats, more work, more resin, more weight. Unless of course you need to cover up the accidental uncatilised booboo hotcoat, something many of us have done.

Thick is exactly not what you want your hotcoat to be.

Lot’s of reasons already stated, and most of them are valid. Lumpy board, lumpy hotcoat.

But, when applying the hotcoat, how is your brusch technique? I think this has not been mentioned yet. Bad technique when aplying the hotcoat will most definitley lead to a lumpy hotcoat. If this is your problem then what you need to do is work on gettting your hotcoat thinner and smoother.

Warming it up in hot water should be mandatory. Warm resin will flow better and create a thinner more uniform surface. I take my container of sanding resin and warm it in a pot of hot water for a minute…mix in catalyst, and then warm again for a minute while slowly stirring, and then you are ready to go.

To me, applying hotcoat is a two stage process with the brush.

Be fast and precise with the brush. Step 1: Pour the sanding coat out in long lines lengthwise on the board and press firmy wiuth the brush to evenly spread the resin into the weave…this should be accomplished very quickly and never going over the same area twice. Walk the brush like it’s a planer pressing the resin firmly into the weave. At this point don’t worry if it’s not the most even coverage, you’ll fix that in the next step. The importantt this is to get the resin over the the entire board.

Step 2: Then take the brush and use lighter pressure than step 1, brush from rail to rail at a 45 degree angle the entire board. Again be precise and fluid with the brush. Then reverse angles and repeat the process. Lastly take the brush and with very light pressure start at one end of the board and brush the entire board lengthwise…start on the stringer for yor first pass and end at the opposite end of the stringer. then move towards the rail a few inches and brush lengthwise.

That’s it. You are making very little contact between resin and brush. The less the better. Warm resin, and proper brush technique will produce a perfectly even hot coat, thus reducing the need to sand down any lumps.


Honolulu, thanks for the warm welcome.

I did a search on UV resin, and it led me to here. I also found that Herb Spitzer (the guy with the angry dingo dog lam) posts here from time to time. I figured if he posts and tries to help out, this place must be cool.

Howzit schoeder,Welcome to the mother lode for surfboard building, do some archive searching for some real treats. Herb doesn't post as much as he used to, but he still lurks in the back ground. If you are serious about the craft then this is the place to be. Aloha,Kokua