Ok I need some help please. When I grind down my laps I will sometimes get marks from the sandpaper. Like the lap will turn yellow if I’m using a yellow backed sandpaper and it’ll sometimes get red marks from a red backed. I’ve switched to a 40 grit grind pad and that is better but then I was grinding down the finboxes and then I got a black mark on the lam from the box. I am getting really frustrated by this happening. Its purely cosmetic but I am a perfectionist and want it to look perfect. Any help would be greatly appreciated! Thank you!
3m sand paper?
…upload photos; detailing the areas; photos showing the sandpaper and blocks and tells more about when you sand the laps; what stage exactly.
I had this problem when first learning which paper and discs to use at which time. I learned that sanding the laps with the yellow 6inch discs would leave gummy yellow colored spots on the areas I was trying to sand and would have to get a new piece of paper on the sander to try to then sand off the yellow spot. I learned two things… I was going too fast and it was heating and melting the resin pretty much. I also learned that the yellow discs were not what I needed. I then found these blue 36 grit pads and they work awesome. Cuts lap sanding time down and keeps it clean. Still go at a low speed but it will not gum up so much. You will need to stop and scrape the build up off the pad every couple minutes, but they are pretty darn good.
Once you have taken down the initial ridge you can go back with less coarse paper to refine it, but you should be good only using the blue discs. This is just what I found works for me before hot coating.
those yellow and reddish colored discs really only work best on hot coats and gloss coats or they gum up in a flash. good luck!
Most commonly used for grinding laps is a Die Grinder and 3M ROLOC disks. Two inch or three inch disks and a backer pad. They are usually green in color. Indasa makes a ROLOC type disk as well. I’ve given up on 3M sandpaper and now use only Indasa/Ryhnolux sheets and disks. 3M has changed the backing of the sandpaper so many times that it’s hard to keep up with them. It takes two or three sheets to do the same job that one sheet of Indasa does. You are bearing down too hard and oversanding. The paper and disk are getting hot. You are burning and melting. Every stage of a glass job is affected by how well you did the one before. If you lay down a nice clean cut lap you will be able to make ONE trip around the block with the die grinder. Maybe a little extra effort at the nose and tail relief cuts. I do my fin boxes with a resin disk and a five inch Makita grinder. You will get residue off of a black box if you grind to hard and get it hot it melts. Go fast and light. Don’t heat it up. Spend too much time it one spot and it will get hot! and melt. Blow it all off with an air compressor. This is another reason that most pro glassers use White Sandpaper like Indasa. Not to mention that it is of the highest quality for the $$$$. Lowel
That norton paper sheds colored powder. Probably by design, to help auto body guys see the scratches. Fine under paint.
There is a roll paper that is tan colored that doesn’t leave the dust.
I think you are right about that. I used to really like the Norton Champaigne sand paper. But after watching so many shapers and glassers use Indasa White and Redline; I have moved on and not looking back. On another Topic; Masking Tape. I got to sample Indasa masking tape today and I am super impressed. As good if not better than 3M 233. About 1/2 the $$$$$&
On cut laps, a trick is to paint the lap lines with sand resin with a 1" brush then grind. Smallest disk, 40-60 grit and very low rpm. Best tool is the air die grinder with 2" disks since you can lower the rpm to whatever you want. The Vixen files (rasps actually) work well on lam resin, but painting the sand resin over the laps first keeps you from grinding the cloth away at the line which shows on color work. If it’s a big uneven and ugly free lap with strings everywhere, work on cutting the cloth evenly and pull out a couple of weave rows to minimize strings.