Unless you have a real good eye, get a good batten so you can mark where the good spots are and where the low spots are.
I would consider re-gelcoating above the waterline, simply because it is fast and cheap
to get a good relatively fair base built back up. Use a contrasting color from what you have, and when you sand back to your color at the high spots with the long board, you will know that your lows are mostly filled. You can spot fill with adtech P77, or even Mat and polyester before gelcoating in the worst spots. Then you can skip most of the bulk epoxy fairing, and do more priming.
The best part about polyester, is that once it has reached peak exotherm the vast majority of its shrinking is done. Not so with epoxy... You've got a 2-3 month cure time before everything is absolutely done, and then each time it gets hotter than it has in the past it post cures a little bit more.
Don't do the notched trowel bit with epoxy. Works fine with polyester as the cure on polyester is pretty much done with the shrinkage. I've done the notched trowel "sand the boat out in half the time..." only to spray finish primer and sand it out a few weeks later to see zebra stripes where the two different pulls were shrinking at different rates. You can do it, but the boat needs to sit outside for a summer after you've got her smooth, then be sanded out in the fall before painting.
If you spot fill the worst pockets with no more than a 6 inch trowel and sand out the boat, you won't put in very many large high spots. The wider the trowel and thicker the putty goes on the higher the chances that the first days putty work doesn't get sanded all the way down... meaning the entire rest of the surface has to get built out to that high for it to sand out. Visualize an 1/8th inch thick pancake 12 inches in diameter. A 12 inch trowel can put that on the side of the boat in an instant. All it takes is a moment of inattention and putty that has started to kick. If you don't sand ALL of it off, you say: "hey there is a low spot around this spot... I better fill that..." Directly the whole side of the boat is built out and when you spray topcoat it looks like its got a knot
on the side of its head
So: If the boat was fair and slick and you removed material before the blasting... you don't have high spots. Don't make any! Use your batten and mark where it shows light... then don't paint
outside the lines.
If all you have are low spots, you can screed the hull, which is my favorite way to fair things.
If you can get 2 friends that don't mind getting destroyed to help out, you can take a sweet bending straight grained fir batten about 10 feet long and screed the hull from the waterline up, for the bow and stern, and a 3/4x3/4 batten for the tight round amidships.
I use Awlgrip Awlfair, but Alexseals grey putty pulls smoothest.
Mark out the low spots first, with the same batten you will be pulling the putty with, and a bright light. Where ever there is an air gap mark the outside edges where the batten contacts the hull.
Once you have all the surfaces marked, mark vertical lines where the end of the batten rides, and leave them naked... no putty so you have an index surface incase you need to do it again.
Putty the low spots with 1/8th or 3/16ths of putty and pull the batten slowly perpendicular from waterline to shear as high as you can before rotating the higher end up to meet the shearline.
It will look rough where there are low spots. Scrape the batten clean, load it up above the low spots and spot fill with a small trowel and pull once more.
If it starts acting the least bit gummy (use slow hardener) stop, and move on... Let the rest of the stuff kick off, even if you mix to much.
Clean everything, and move to the next length.
Once the boat is done, stand back and look at the gloppy mess you've got.
The next day...
Sand with 3M marine
4 1/2 inch wide boards, and 16 inch 2 1/2 boards with Mirca Abranet on them. Use 3M dry guide coat, and work from either the stern forward, from the Shearline down to the waterline holding the board flat to the boat, meaning the board has to walk and tilt to stay in contact. 45 degrees... When you make it to the boat you should have an even scratch pattern texture down to the gelcoat
where the surface was good. Reverse direction and go from the bow shear down to waterline until you meet the transom.
Now you should have a boat that is totally covered in X marked scratches with a bunch of pock marks, holes, and shiny stripes where putty did not make it.
I normally start with 40 grit for the first pass, then go over the boat after another round of dry guide coat with 80 grit so you can feel any waves and ripples.
Now that she is smooth, hold your battens back up (need a helper) and eyeball once more. Mark out the remaining lows.
Once the lows are marked, go around the boat with a dremel or die grinder with a pencil burr and poke each of the air pockets or pits and mark with a pencil. Take a wire wheel
on a drill to the streaks and lows that trails the batten left.
Now with the batten, fill the low spots. Follow around with a small putty knife and fill the pin holes... Then follow up with a 6 inch wide flexible trowel and hit the streaks.
When you stand back it should look like a camo paint
job, the sanded surface is light pink the new putty is red. Don't paint outside your lines... don't put any extra putty on the boat.
The next day, longboard the low spots and check with the batten.
If everything is true, you can either short board the remaining small spots or take a DA sander to the hull going from waterline to shear and back down... and back up, and back down... in passes that overlap about an inch. I do a 2 foot x 2 foot area, and overlap into the next both top and bottom until the boat is done. If the sandpaper is sharp after the first 2 square, then keep trucking. I like 3M gold 80 grit on a hookit pad. At this point you are not sanding
down to the lows, just taking off any excess that may be present from the filling.
No bulk material removal
After this, I use Awlgrips brown sprayable fairing compound. You need to 545 the boat as a tie coat but can go wet on wet with the fairing compound. You can roll it, but spraying it is best.
Before you do, another round with the batten and a study with your finger tips will show where you have a wave or ripple. These areas get one or two coats, before the boat gets a full cover coat.
The next day, sand a small area and if it smells like solvent let it sit another day. Depending on thickness and humidity it takes awhile for the solvent to evaporate out. If you sand it while it smells like solvent then the thickest areas will shrink after the solvent flashes off.
If you roll, I start out with 40 grit to take off the roller stipple, then go to town with 80 grit until a haze of pink begins to show through. Stop before you sand down to the pink.
If you have dark brown shiny spots, you've got low spots that the long board can cross over... which means that some point someone got feisty with that particular area...
I usually fill those areas with pink putty, and continue on... For spots smaller than a foot square, I use an aluminum
yardstick as a screed. You can warp it to the shape of the hull, and also rub it against the hull to leave marks. Works well once the rest of the boat is fair.
Get the stem, transom corners and any detail around molding or cove lines absolutely perfect. I'll go so far as to sand out these areas to 180 grit while still in the putty stage. Whatever you can do to make inside corners not be a high spot, and radius to be smooth with no low spots will greatly be rewarded by your finger tips. Even if it means spending an extra day or two filling and fairing only details and radius, you will be rewarded.
If the boat isn't fair, and isn't smooth.... primer is the most expensive fairing material there is!
You can sand out the whole boat with 120 grit, but by the time the 80 grit scratches are gone you have sanded off the sprayable fairing. I argue with myself over this...
I normally spend a day and spray Awlquick. As soon as the dew flashes in the morning, the pressure pot comes out and a full wet coat goes on. When the recoat window is up... another full wet coat goes on. Rinse, Wash, Repeat.
The goal is to bury the 120 grit scratches, and give enough body to be able to sand out the boat. If the boat is exceptionally smooth, I will start out with 320 grit on a DA sander and dry guide coat. I take off the roller stipple and overspray with 220 grit, and once it is smooth to the touch, switch to 320. Often 320 grit cuts finish primer faster than 120/180 if the humidity is down... It dusts up and disappears instead of filling the sanding
Any details and corners, use a square 1/4 sheet vibrating sander to get as close as you dare, follow up with a rubber block. Don't pick at any particular area, you've got to blend in what is good...
If you feel like you are going to go to far, and burn through, use red scotchbrite to rough up the area mark out a fine pencil line and spot prime the area. If you can't sand down to roller stipple and overspray, the area is a low spot... so don't sand down to roller stipple and overspray! Scuff it, mark it, and prime it.
At this point you can also use denatured alcohol and food
coloring to stain the primer. I like green. Basically toss a 1/4 ounce in a pint of denatured alcohol and drop a rag in, then go to town wiping down the boat. When the texture is gone, so is the green. I like to take a pass with the 16 inch board and 320 on it, over the green once as any low spots missed in previous cycles jump out as un-scratched spots.
When you get smooth, you will still have pin holes in places. I use Evercoat Ultrasmooth finishing putty with blue hardener and apply it with a razor blade. You can also use Awlquick and a squeegee, but that is sand tomorrow territory.
Go over the entire boat with a fine tooth comb. If you don't have an aircompressor, you need to do a denatured alcohol wipe with two rags and flash off a gallon or so over the whole boat to flush the dust out of the pin holes so you can see them.
Fix everything, block sand with a rubber block and 320... then spray or roll 545 over the boat. If you are already in 320 grit in the awlfair stage then you are smooth already and don't have any highs or lows to hit, to burn through the 545. A full coverage coat with 545 is still fairly thin.
Automotive painters think boat guys are nuts for using grits like 220 on finish primer. They have a point. The scratch profile is huge... if it were not that the LPU topcoats can cover 320 grit we would be doing just the same as they do with their similar epoxy primers... Finish primer, 320 takes off the overspray and go on up the grits.
220 grit is closer to your driveway, than finished topcoat!