The purpose of sanding before your final coat is to flatten and smooth out the entire board and remove any high spots so you do not find them during your final sanding. The dull sanded surface is also required for a strong mechanical bond between the now cured hot coat and the gloss coat. You should use a 100-grit paper to flatten the surface as it is aggressive enough to quickly level the surface without too much chance of exposing the weave (burn through) If you use dull sand paper or too fine of grit you will heat up the glass fabric and the weave will leave a weird shadow in the final product.
Professional shapers use a variable-speed polisher with a hard Flex Pad to quickly level their boards. This is a slow speed polisher not a high speed angle grinder! The large surface area of the Flex Pad creates a flat surface quickly. These sanders are awesome in the hands of a professional but if you have never used one mistakes happen fast. It is essential that the sanding pad is held flat at all times and you are using a fairly slow RPM. The sander goes up to 3000 rpm but it should be used at 1500 or lower for this step. The faster the pad spins the more aggressive the removal rate and the more heat you are producing. It is very important to keep the sander moving in a smooth motion as couple of passes is all it takes to level a board. If you tilt the sander or stay in one spot you will quickly burn through and expose the glass. I do not recommend doing the rails with this style of sander unless you are very experienced and have a very light touch.
Most homebuilders will not have a 7″ variable speed polisher and a hard flex pad so the rest of this tutorial will be about sanding your board with a 5 inch orbit sander. The orbital sander will take 2-3 times as long to complete this step but it has much less chance of causing problems. If you factor in the time it takes to clean-up the mess from the larger variable sander (no dust collection) it is probably faster to sand a single board with an orbital sander anyways. A regular Shop-vac hose can easily attached to an orbital sander to prevent the dust from getting everywhere. This set-up is a bit cumbersome but if you have to sand indoors it is worth doing.
FIN BOXES AND LEASH PLUG
The first step in sanding your board is to sand the fin box and leash plug flush with the rest of the hot coat. Use a medium grit (60-80) sandpaper to sand your fin box flush with the bottom of the board. This is the only time it is ok to tilt your sander for more aggressive removal but keep it moving back and forth along the box. With the sander tilted you can hog off the bulk of the excess before finishing with the sander in a more normal flat position. Remove the sander entirely every 20 seconds or so to let things cool down as you are trying to sand off the excess material not melt the fin box.
As you sand your board you should always be watching for signs of the weave starting to show. The cross hatching of the weave will show before you actually sand right into the weave. As soon as you see weave STOP. If you continue you will be opening the weave more and you are already too low. Burn through happens and is not a big deal with the gloss coat to follow but ideally you will not have any spots like this. If you mistakenly sand through on your final sanding you have to seal the weave or it can absorb water.
SANDING THE TOP & BOTTOM
Once your boxes are sanded flush it is time to sand the top and bottom surfaces. 100-grit in a 5″ orbital seems to be a good balance between removal rate and minimal chance of burning through or over heating the resin. The goal is to remove the entire glossy surface leaving a flat even dull surface for the gloss coat. The better your glass job was the more enjoyable this step is!
Start on the bottom of your board as it is flatter and easier to sand. Starting in one area and you will quickly see the high spots (shown) flattening out. Hold the sanding pad as flat as possible and work your way over the whole surface removing the shine. It is best to let the sander do the work and not add a bunch of hand pressure to get faster results. If you have done a poor glassing job it is better to use a more aggressive paper than to press down on the sander to achieve faster removal. The objective is to have a smooth surface so don’t chase low spots by tilting your sander or keeping the sander in one spot. The sander should always be held flat and kept moving.
Sags sometimes happen where gravity and a thick coat of resin were applied. The sags are easily fixed by carefully sanding the high ridges until the lows are dull. This picture shows the first pass over a sag that will be sanded more.
This area has been properly sanded and it is very easy to see the shiny low spot. If you have a bunch of really low spots it might be wise to add a drop of resin transforming them into pimples before you do the gloss coat. If the low spots are close to the surface you can just scuff them by hand and the gloss coat will level them. If the low spots are on the rails it is advisable to add the drop as gravity will be working against you. You do not need to achieve perfection so after you have sanded away most of the shine from the bottom, flip the board over and start sanding the top.
SANDING THE RAILS
The objective is the same same on the rails only the removal rate is much faster. The contact area on the pad is very small so burn through can happen fast. It seems to be easiest to connect the dull top and bottom sections until the shiny rail section is evenly dull and then move to the next area. Work the sander back and forth with slight changes to the the pad angle with every stroke. Working 8-10 inch sections at a time seems to make quick work of the rails. Flatten the surface and remove the shine, then move to the next section. Try to move the sander quickly as the rails do not take much sanding.
Tip: If you have a pile of used sanding pads you can reuse the less abrasive pads on the rails. This is the same as switching to a less abrasive pad 120-grit or 150-grit pad.
The nose and tail areas are the hardest parts to sand with an orbital as the sander is removing the most material. There is no shame in doing these sections by hand especially if you have a vacuum hose attached to your sander. The dexterity with a sanding block is so much better with a small sanding block that it is a wise choice for the delicate spots. You may have a thin line along your rail from your tape barrier during the hot coat stage that can also be touched up with the sanding pad. It is best to sand this line away as it really sucks to find any high spots when you are sanding you gloss coat.