Scuffing up the low spots is required, the only tooth that fresh epoxy
has on old epoxy
is mechanical... and if there is any amine blush on the surface you have a scab that wants to fall off. Epoxy doesn't crack off often, but when it does there is normally a missed "shiny" spot under it.
I normally go from 80 grit to 220 grit. 150 and 180 are a stop along the way only if I've got a product that sands easier with them, or as a test in long boarding before 545. Some of the harder high build primers (ultrabuild awlgrip, I'm looking at you) nearly require it. The smoother sanding
stuff, doesn't need it once you are fair and flat.
Something you can do, after you pull putty and see that you have a low spot is to wait an hour or so and mix up some 407 to ketchup consistency and coat over the low area you see and fill in the texture/holes in the putty job. The color is different from peanut butter and ketchup, but you can brush it on and put a slick over the filling work
so you don't have low spots in what you are trying to fill up. You've got right about 5 hours at the maximum with west systems to overcoat and have a chemical bond before the amine blush gets you.
Something I like to do, is to roll on a coat of resin at the last of the 80 grit where you are deciding whether you want to prime or not. I don't normally sand much in the inbetween grits... With epoxy and high build primers you can normally jump successfully from 80 to 220 once you change products on the surface.
There is a little math involved, as you need 15 mils to fill all the 80 grit scratches that can hang up a finger nail... and enough over the top of that to sand off any stipple and texture. That is a lot of material, but the thing about evenly applying a lot of material is that you have something to sand back without burning through onto your high spots, and enough material in the low spots that you've done some filling work
in the process.
I normally sand off the first of the stipple of the first heavy priming/cover coat with 80 on the DA until a haze of stipple still shows (3m dry guide coat) and then switch to 220 and work it down the rest of the way. From there each coat sanded by 220 can be 2-3 mils dry after the stipple is off and you are in business... Then put 180 or 220 on the long board apply another round of guide coat and see what the surface looks like. If you are smooth to the DA in 220, you are normally real close on the long board in 220... If not drop back to 180 and cut until you are.
If you have enough material on, sand until done.
I wait until 545 before breaking out the 320 unless I'm making flat panels
to be installed. Then it is resin coat, sand in 80 and Awlquick 3-4 coats straight to 320 on the mud hog on a flat work bench. Done.
I like the resin coat if cost is a big concern, as it is 100% solids and is self guide coating for low spots when you try to sand it off and will let you know exactly where your DA can't sand. It does a good job sealing exposed putty, and fills most of the pin holes better than trying to prime them alone. You can do small spot repairs
with 5 minute epoxy and an acid brush on detail areas if you want to try to prime today, and not pull another round of putty. Outside corners are nice for that, as normally a low spot on them is a thin brush stroke, but until you put something there they stay a low haze the whole way through priming...
I'm not a huge fan of 410 on anything except inside corner fillets, as it is soft stuff with any thickness... You get dents to go with dings, where 407 is tougher to dent. Claw
... If you are working on something and drop a claw
hammer on deck
, do you have a problem?
You also end up with halos around the 410, as it is softer than the 407... so if/when I use it, I tend to do it only hand sanding
with a piece of foam as a block.
Blue dow foam, 1/2 inch thick cut to 8 inches or so in length for a piece of sticky backed 80 grit. It is self conforming to a bit, after an hour your finger tips have holes to grab, and the edge you are smashing into the fillet, fits the fillets shape. To get no lines cut into the fillet, you sand 30 degrees or so off the fillet, into the fillet until you get a cut that is smooth along its length. Any shiny spots are low spots. When you shoot top coat over that, there isn't a cut line above and below the fillet like if you use a PVC pipe or something else round.
If you are spraying, once you have the fillets caught up... I like to shoot just the fillets the first pass 2-3 inches to each side and then shoot the rest of the boat
. The reason is the fan of the spray gun has a hard time blowing into an inside corner and depositing the same amount of material as it does on an outside corner or flat surface. The fillets can be perfectly sanded now, but if you want to blow on three or four heavy coats of primer they become a little bit low at the end of the sanding.
Red scotchbrite also has a place in fillet sanding, as you can spot prime areas and scuff up what needs to be scuffed, but all the overspray has to come out of the fillets or it looks like astroturf when you shoot 545 over the top. When they have shape, red scotcbrite removes material evenly and you don't have to sand and block them out as hard each time you shoot the boat
The nonskid will hide a lot, but what it does not hide are hard edge lines around putty fills, as they show as a crease... and they also do not hide overlaps and edge lines in the fiberglass
. Unless you put enough high build primer on the whole surface to bury those too, which takes a lot of material. You can spot prime those spots to level them out, if you want to make some time without the long board. They are high spots (hence the crease around the edge) but you can make them smooth out enough not to be noticeable in the primer. Hand size low spots are also tough, as they hold dirt and show themselves.
If you are rolling, I'm beginning to like Interlux
Interprotect as a high build primer/ sanding base simply because of the overcoat times it allows. Once every 2 hours, is about the same speed as spraying rather than once every 12 when rolling 545 and Awlquick. I've found the 2333N thinner works a lot smoother at 10%. It isn't great sanding stuff, and rolls like astroturf... but for burying the putty work in a day I can get along with it for the 80 grit make her white jump before 545.
Spraying I use a pressure pot and a couple kits of Awlquick to bury the putty work.