Sanding, Sanders, Sand paper... I've Sanded Enough Already!

90% of all boat work, seems to involve sanding.

Of what involves sanding...  Seems to involve sanding the same pieces, repeatedly with little visible change until they are painted.

The irony of Re-painting something that has already been painted, is that unless the existing paint is the same type as what you are painting with, and in relatively good condition it ends up coming off...  Which means sanding with a rough grit, over an area that has been gone over thoroughly in days gone by.

The difference between Sanders, make a big difference in the amount of time it takes to do a job.

Dual Action and Random Orbital Sanders spin and oscillate with a path that doesn't put inline scratches into a surface and makes it easier to get something smooth.  A grinder, or polisher/sander that doesn't take a different orbit can scar the surface you are working.  That may or may not be a big deal depending on how far you are along in the painting process. 

For bulk material removal, and general shaping I like a Makita 9227c with an 8 inch vinyl pad.  They remove a lot of material quickly, which can be a very good or very bad thing. 

 If you have scratches that are in a straight lines, or circles the next finer grit follows the same way and keys into them and they don't go away.  That is why a coarser grit on a R.O.S. looks like the next finer grit, on a hand block.

Of the Random Orbital Sanders, there are a few differences...  You can have square pads, or round pads, and even long board pads.  There are differences between the size orbits the pad moves, a fine finishing sander may be 3/32nds, while other sanders have a 3/16th orbit and even in very fine grits will show rings in the finish.

I'm going to assume at this juncture, that you are equipped with only electric supply and not a large air compressor suited for keeping up with pneumatic tools.  If you have air, I like Dynabrade DA's, and Hutchins inline sanders. 

I've found that the Dewalt product line does a decent job for a relatively inexpensive cost when only electricity is available.  Most of the time a 5 inch random orbital sander fits where you need to sand on a boat, though a 6 inch pad does a flatter job. 

The Porter Cable 6inch Orbital sander, is another companion...  They don't last all that long if you put them into 8 hour days, and lean on them at all... but do a lot of work due to the larger orbit and greater power than 5 inch sanders have.

1/4 sheet square pad sanders have a place too, as they vibrate.  Vibrating means that you don't have swirl marks.  On small parts, and inside corners they can make lighter work than what would otherwise need to be hand sanded.  I use 80 grit, and 220 grit almost exclusively on these.  80 grit for paint stripping, and fairing in putty/high build primer.   220 grit for all finish sanding and paint de-nibbing.

As far as sandpaper goes, cheap sandpaper doesn't last very long or cut as fast as decent paper.  3M gold, is about the price point where you are paying for what you get, which is... decent sand-paper.  I use 3m Gold from 80 grit through 320 grit, in a combination of hookit backing, and sitckit depending on the tool. 

In 40 grit, you can spend a little more and buy Norton Blue-Mag, but for most work Green F weight 3M production paper works good enough for the cost.





    


 

Information for the DIY boat builder about how to get a good paint job.

The common wisdom, is that a paint job is 90% prep and 10% application, and it is true...  But that only sums up half of what goes into a paint job.

The other half of painting, is experience of knowing what to do and when the prep work is done at each phase.  I'm writing this to try to share some of my experiences, and hopefully save you some time.

The biggest part of a good paint job, is deciding what level of finish you want to aim... and studying the hull very closely to see where it may have issues...  In addition to just dock bruises and scrapes.  You can make a very shiny hull, that shows a lot of waves and shapes, or a very fair hull that has a rough finish. 

To get a perfectly smooth paint job, it requires a fair bit of long board sanding and screeding on filler, as a fiberglass boat may have come out of a mold perfect, but over time the stresses and heat cycles starts showing bulkheads and tabbing lines.  If you have a dark colored hull, this is easier to see, as the heat cycles post-cure the hull and shrink it around its bulkheads showing them as though they are a rib cage.  If you have a white boat now, and are going to paint it dark blue or another dark color, you need to be aware that the initial finish may be perfect but the boat is going to post-cure and move a bit after its first summer.  It may be worth doing a first pass, paint job to make her dark and lay a foundation for later fairing...  Otherwise you may spend a lot of time and materials, that will not look quite so perfect just due to the nature of boats. 

So, sighting down and around the boat looking for bumps, lumps, and low spots is required if you want a fair, and shiny finish.

If you are dealing with a boat that has already been painted, you need to figure out what kind of paint is on it...  You can find out a lot by taking the thinner, T0006, and a box cutter and scoring the hull over a 2 inch square somewhere close to an area that already has docking scars.  Hold the rag to it for a minute or two, and if the paint lifts you need to take the boat back down to the gelcoat.

In most industries, be it a boat, motorcycle, or car...  They sell better with a pretty paint job, and a pretty paint job with poor prep work is a "Scuff and Puff."  You may have an Awlgrip top coat, and a receipt for the labor and materials...  over an enamel painted boat.  This should be a sin, as the enamel paint under a two part polyurethane starts to suffer from hydrolysis and the water vapor pressure that the porous gel-coat, and porous enamel want to release, gets trapped under the not-so-porous two part Polyurethane.  Where the enamel lifts you get little brown spots under the first layer of paint. 

Finding a job like that, is a sad thing if you want the boat not to continually show blisters in its paint, because once you know the top coat is awlgrip, people will paint and repaint fixing blisters as they come season after season and then you really do have a mess to take back down to a bare hull.

Awlgrip is tough enough that water based paint strippers don't touch it, so you may end up sanding the top layer off if you want to chemically strip the underlying enamel.  I use Aqua-Strip around cove stripes and areas with a lot of detail, as you can save a mountain of hand sanding time...  Most of those areas have poor adhesion anyway, due to the shape and minimal fillet radius.

The next part of this series will be about sanding.