Part 6: Sanding your Glass Job

In our opinion, sanding is the most underappreciated and also the most difficult step of board building to master. Once again, you can go a long way to shorten your learning curve in sanding with good tools and technique, but there is no substitute for practice. Like shaping, you could do an entire sand job by hand, but you will get better, faster and more consistent results if you use a variable-speed power sander. Your sander should have speeds from about 0-3,000 RPM and accept 5/8” course thread sanding disc pads, such as the Power Pads that are very popular with experienced surfboard sanders. For a proper sanding job, you will need to have a hard or medium pad and a soft pad. As for sandpaper grits, you will want some medium grit (60-80) sandpaper or adhesive-backed sanding discs for your hard/medium pad and a progressive range of higher grit sandpaper or adhesive backed discs (120, 150, 220, 320, 400) for your soft-pad. You will also be doing some hand-sanding in the sensitive areas (rails) so you’ll want to have some soft sanding blocks with paper in the 120-400 grit range. If you are just using regular cloth backed sandpaper on your sanding discs, you need to cut the paper in the circular shape with scissors, and some temporary adhesive spray (3M Super 77) so stick the paper to the sanding pad. Finally, you want stability when sanding, so use your shaping rack (not glassing stands) for your sanding job.

 The purpose of sanding your glass job is to flatten and smooth out the entire board by sanding off the shiny gloss finish, then to slowly build back the shine by sanding with progressively higher grits. Typically you start sanding by flattening out your fin boxes with your medium/hard sanding pad and 60-80 grit sandpaper. Before you even do this, however, it is a good idea to put some 120 grit sandpaper on your soft pad, and do a quick pass on the deck of your board in the areas where the board will be contacting your shaping rack. This helps keep the board from slipping on the rack while you are sanding the bottom.

 Sanding Technique

Power sanding effectively is all about technique. As a beginner, you can always reduce your RPM. The slower your RPM, the less material the sander will remove. On a six-speed (common) sander, you really don’t want to be sanding much higher than level 3 (1,500 RPM) to be safe. Try to hold the sanding disc as flat as possible as you are moving it around the board. If the pad is on too much of an angle, it will dig (especially hard/medium pads) and create a rut. Keep it flat, and keep it moving. If you stay in one place with the sander, you will create a dip or “burn” through the weave in the fiberglass. Another trouble spot is the curved areas, especially the nose and rail areas. You really want to reduce your RPMs in these areas and keep your pressure lighter. If you are uncomfortable with the sander, the rails and nose areas are good places to hand sand with a soft sanding block. Better safe than sorry.

 Flattening the Fin Boxes and Leash Plug

You want to use your hard/medium sanding pad and medium grit (60-80) sandpaper to sand your fin boxes flush with the bottom of the board. Use low-medium RPMs and don’t continually press down with the sander in one place. Keep the sander moving back and forth on the fin box, and remove the sander entirely every 20 seconds or so. If you have constant pressure, you run the risk of over-heating and distorting the fin box. Try to remove material as evenly as possible by keeping the disc moving from front to back, and holding the disc as flat as possible while you are sanding. Keep grinding the boxes down with your hard/medium pad until they are only about 1/16” above the surface of the board. You will get them totally flat when you move on to your soft pad and higher grits.

 While you still have your hard/medium pad in hand, you may as will flip over the board and grind down your leash plug. Again, take it down until it’s just above the surface of the board. You will flatten and smooth it down later with your soft pad and higher grits. Keep the sander moving and flat, just like with the fin boxes.

 Sanding the Flats

Once your boxes are sanded flush, its time you load up your 8” soft sanding pad with 120 grit sandpaper and start sanding the flats of your board. The goal is to remove the entire “shiny” surface with your 120 grit. The better your glass job, the easier this job will be. If you have a lumpy glass job, sanding will be more difficult and time consuming.

 Start on the bottom of your board along the stringer line with your sander on a medium speed (setting 3 is good). Hold the sanding pad as flat as possible with medium pressure and work your way nose-tail to remove the shine. You will get a feel for the most effective pressure and sander speed as you go. Just make sure you never stay in one place and keep the sander moving. You should also remove the final bit of fin box material sticking up from the surface during this phase, getting the boxes perfectly flush with the surface of the board.

 Once you have sanded away most of the shine from the bottom, flip the board over and start sanding the deck. Again, be careful in the nose and rail areas. There is no shame in sanding these areas by hand.

 Sanding the Curves

Novice sanders are better off sanding the nose and rail areas by hand, especially with the rougher (120-150) grits, which remove material more quickly and can make gouges in the board if not handled with care. We recommend a soft sanding pad with 120 grit sandpaper to conform to the curved areas. The goal is the same: flatten the surface and remove the shine. The first grit (120) is the toughest of all stages in the sanding process, as you are using the most muscle and removing the most material. If you have any minor dips in the flats that are still shiny, you can try to sand them with the soft sanding pad by hand.

 You may have a thin line along your rail formed from your tape barrier during the hot coat stage. You can smooth this out by scraping it with a razor blade working nose-tail along the rail. Another really nice aid for smoothing out the rails is soft-backed sanding sponge, which wraps the rails smoothly and leaves a nice finish. We suggest you play around with all of these soft-sanding hand-tools to see which ones you like better. Stick with 120 grit until you have removed all of the shine from the board.

 If you plan to do a final Gloss Coat on your board, you should skip the Burn Through and Fine Sanding sections and go right to Pinlining and Glosscoating.  If you don’t plan to do a gloss coat, then your next step is to deal with any Burn Throughs and then Fine Sand your hot-coat.

 Dealing with Burn Throughs

Once you’ve sanded the entire board with 120 grit, there is a very high likelihood (99.9%) that you have some areas of “burn through” where the weave from the fiberglass is exposed. If left exposed, this weave will suck water into your blank, so it needs to be dealt with before you go further. If your board is completely pocked with burn throughs, your best bet is probably just to do another complete hot-coat. This hot coat should come out better and flatter since you’ve already sanded the board pretty smooth. If you are fortunate enough to only have a few burn throughs, you can probably just “spot” hot coat those areas by painting on a thin layer of epoxy to re-seal those spots. Mix up a small quantity of epoxy (don’t forget the Additive F) and use a smaller chip brush to paint a thin layer of epoxy over the burn throughs. Just like a hot coat, cover all of the burn-throughs, then just let the epoxy sit, settle, and cure flat.

 Once these areas cure, you can hand sand them flush by hand with a soft block and 120 grit. Take your time and be careful not to burn through again, and try to feather the edges of these patches smoothly into the existing glass job.

 Fine Grit Sanding

Once you have re-sealed any burn throughs, it’s time to go back to the sander with progressively higher grits of paper to restore the shine back to your glass job. The next grit is typically 150. Sanding with these higher grits should be much easier than the original 120, because at this point you are not trying to remove material. Grits 150 and higher essentially just smooth out the scratch marks from the grits immediately before them. You will be creating a lot of dust while sanding, so it is a good idea to wipe down your board with a rag and denatured alcohol between grits to see if you have done a sufficient job of smoothing out the surface. Again, it’s OK to use soft sanding pads to smooth out the curved areas, just make sure you use the same grit sanding pad as you are using with your power sander.

 You should run your fine grit sander through a progression of finer grits: 150, 220, 320, and finally, 400. Wet sanding with 320 and 400 is also a good idea, since the water keeps the paper cleaner and gives you a more consistent cut. Once you are finished sanding with 400, your board will have a finish comparable to most matte-finish boards you see in surf shops. It is perfectly fine to stop at this point and consider your board FINISHED!