Hello there all. I’d like to thank you all in advance for any light anyone can throw on this subject…
So, I am suffering from a recurring theme when installing my FU longboard boxes. and that is when I grind them down Im finding I have a hairline crack down the side between the box and my resin. Why would that be?
9’3 single fin, PU, poly, 6oz cloth, 10.5" FU longboard box.
So my procedure goes - after my hot-coat is good and cured I route out my fin-box hole using a home made jig and a plunge router. Then I sand down and acetone my fin box. Then I cut a piece of 6 oz to go in the hole and get my dummy fin in fin box ready. My dummy fin is just a regular fin with a notch cut in it so i can insert it in the fin-box with the little box tab complete. ( the notch bridges the tab ) then I mix up straight lam res ( 3 oz ) add a little white pigment and catalyze at 1% ( 15 drops ) I then pour in the hole, push my fin-box down into the hole , taking the cloth with it. Resin splooges out , i wipe, center the fin and tape in place, and leave.
When cured I then then grind down the box with a mini grinder till a 16th over the hot-coat, switch to a pad and 80 grit to finish. Then I notice it has pulled away ,… Why?
I am then left with a tedious and fiddly job of fixing this and capping over it all with a 6 oz patch.
Any help regarding this matter would be greatly appreciated, Cheers.
Perhaps you are heating it too much when sanding it with power sander.
Do it in stages. Touch the sanded area with yout hand, if it is uncomfortablly hot then go significantly slower. Use sharp new sandpaper and beware of heating.
Two things to try, either push your glass down into the routed hole first, then pour. Or skip the glass in the hole, And add a thickener such as cabosil to your resin mix, paste the sides of the routed hole using a stir stick /tongue depressor. Texture should be in the mayonnaise range. Either your resin is draining due to thin viscosity, possibly into where you trapped some air space with the glass, or you are overheating the box when grinding. Maybe both. Good luck and your mileage may vary…
perhaps the resin mix went off a bit ‘‘hot’’ causing the shrink-back, from the box. If the gap is in the hairline range, a good fix can be done with super glue. The thin super glue will wick into very tiny cracks. After it cures, it sands easily. Just a thought.
I’ve had that happen a few times. Not so much after I started using a 36 grit ‘Rol-Lok’ disc to prep the outside of the box. Epoxy seems to work a bit better in adhering to the sides of the box also. I have always ‘capped’ my boxes with a patch so no big deal… the separation line just gets filled during the capping process anyway.
My vote is for too much heat. I used to use a fin system similar to ProBox and I had problems early on with the boxes getting very hot from friction when I would grind them down flush to the bottom of the board. Then I’d get gaps around the edges. The plastic would get quite hot and we all know that things expand when they heat up and then contract when they cool. When the box would swell it would push the glass back with it and then when the plastic box would contract the glass wouldn’t come back with it. I actually found that if I used a 40 grit grinding disk instead of 80 grit on the sander there was a lot less heat and it resolved my problem. Shortly thereafter I abandoned those fin boxes.
Does anyone (else) cut down their boxes to a little proud of the bottom glass before hitting them with the sander?
Japanese pull saw, flush cut saw, oscillating multi-tool, router…
Dummy fin pushing out on box, like mataco said. This was the first thing I thought. This issue combines with . . .
Polyester resin shrinkage, which is fairly significant. And, the bigger the space your filling, the more pronounced the effect. Also, I don’t know which color box you are installing but . . .
White boxes have more adhesion issues than black ones. I was told this years ago by Fins Unlimited themselves.
Possibly sanding too early / with too fine a grit / getting hot.
Epoxy will reduce the effects of points 2 and 3. Just beware of runaway exotherm.
I had this issue on my last fin install. I use the exact some process as you except I broke the little tab out of the box to insert my dummy fin. I will cut a notch out of the base of my fin like you do and maybe this will help. I used super glue to fill in the little gap like Thrailkill suggested. I’m gonna grind that thing down soon and see if it holds. Definitely getting a cap on top.
When you pull out the dummy fin in order to sand the box, it’s a good idea to insert a piece of foam in the slot, forcing it down as much as you can. this will keep the box from shrinking when you sand. Works with me.
I’ve had this happen with PU/PE builds, but it’s been a long time since I did one of those. I can’t remember if I used laminating resin or sanding resin when I installed the box, or if I scratched the surface of the box like I do now. I did a small glass cap to seal it.
I haven’t had this problem with the EPS/Epoxy builds. I do move the grinder around a lot when I sand down the box after installing it to keep from heating up the box or foam. I also do the glass cap now almost all the time. It makes sense to have the layer of glass over the box.