Sanding a mile of scratches

Thread starter #1
So in the off season, I'm trying to bring my faded, scratched up 75 laser back to it's firey bright "70's Orange-Red" glory. I pulled off my old vinyl CF numbers and a harbor sticker from 1978 to get a view of the original color. 70's retro is in, right?

Naturally, I started with Gouvernail's "How to make your bottom pretty" article at To Make Your Bottom Pretty

Now, the Gouv talks about "Every scratch that you can feel will need to be filled." Uh, riiiiggggght.

You've gotta understand that I've got a boat that sometime in a former life clearly needed to be dragged over a beach every time it went sailing. My best guess is that if I laid out all the scratches end to end, I'd have somewhere between 1000 and 5000 linear feet of scratches -- and this on a 14' boat. (I'll try and put up some before/after pix)

So I've got a brand new 5" random-orbit sander dedicated for the job. I started in on the 320, just to see how it would go.

Maybe we need some 220.

Maybe we need some 120.

Maybe I should jump to the 80.

OK, so I'm sanding the thing out with the 80 grit, and I'm starting to reduce the number of scratches to something I can possibly imagine maybe trying to fill. I've done maybe half the boat in a couple hours. But I'm burning through lots of 80 grit discs, and lots of fairly superficial scratches remain. And that late-fall sun is heading for the horizon.

Hmmmm. ...still lots of gelcoat depth left.

I've got a couple sheets of 40 grit lying around. Maybe I'll try a small area, just to see.

Maybe it'll work.

Maybe I'll grind straight through to the glass and make even more work for myself.

But I tried it out, and yep, for this particular boat that's gonna be the ticket.

Now, my laser was built in 75, and it's possible the builder was using a thicker gelcoat layup back then than they do now. I can see how thick the gelcoat is by examining a couple of chips that go right down to the glass, and it's clearly a LOT deeper than the gelcoat on the deck.

So starting next weekend, I'm going to flip it over again, and go to town with the 40 grit. I figure I need to take about half the gelcoat down in order to have a prayer of getting a decent looking finish. After I'm done, I'm hoping there will be less than 100 linear feet of real and serious scratches to fill. Maybe a lot less.

Yeah, I know; Fred says fill first, then sand. I figure I need to do my major sanding first just to find the scratches worthy of being filled in. Then I'll touch up the filled scratches.

To be continued...
Re: Sanding

Holy jesus.

Post some pics. Sounds horrific. LOL

If a R/O sander and P40 is required, the hull must have looked like a mountain range.

I feel all ill.

(gelcoat weighs a ton. swweep up the stuff you sand off and weigh it.)


Active Member
Re: Sanding

Maybe If I back off a little??
Certainly you should fill and gouges where you can see laminate through teh gelcoat.

If you fill the deeper scratches you reduce the odds of sanding through.

Last week we sanded the sides of a 1972 South Coast 21. We blew through about 25 6 inch 60 grit self stick 3M gold discs.

About the 40 grit...I would try sanding a small area with 80 grit.( 3M Gold, 3M Green Corps or Norton Blue Magnum will stay sharp a lot poonger and actually pay for higher purchase price) Time teh sanding and see if you can stand to take that long. 40 grit is mighty agressive and you may force yourself into a total refinish...which is sooooo much work.

Good luck. The last Laser bottom I refinished is in my yard waiting for me to refinish the deck. I started sanding the bottom sometime in 1993.
Thread starter #4
Re: Sanding

Maybe If I back off a little??
... The last Laser bottom I refinished is in my yard waiting for me to refinish the deck. I started sanding the bottom sometime in 1993.

Anyhow, thanks for leaving that article posted on your website. It's a great help.
Thread starter #5
Re: Sanding

I just got off the phone with McMaster-Carr, my favorite industrial/shop supply house. Plug -- if it's a tool, they DO have it, and you'll be able to find it on their website. Price isn't always the most competitive, however.

Anyhow, the finest 5" hook-loop 8-hole sanding discs available seem to be 60 grit, not 40 grit. So I've stocked up with 25 discs of 60 (plus the full course from there on down to 1500) and I'll take over with that.

At the end, I'm going to have to assess just how much was spent just on sandpaper. :eek:
Hi Steven,

I'd fill first. Bondo with some red/yellow coloring comes out pinkish, but it helps. Fred may suggest other fillers, even gelcoat. Then do all that sanding. I use a steel taping knife to push/scrape the bondo, so little extra is left on. It helps when there are too many to count. The bondo(polyester) will shrink, so two coats are better. And it's not waterproof so dry sailed only. I gelcoat scratches when they are countable, thus cleanable.

The time you save can go into bomb=proofing that mast step, thru-hull fitting, Hull/deck joint, etc. etc.



Hey, who changed my topic tagline?
Well, Mr. Moose, there aren't a lot of choices on that one. ;) I did it. There's a standing Bradley directive to make unique titles to the threads, in order to make it easy to find them with the Search feature.

A mile of scratches is an exaggeration, of course, but if you've got a possible 5000 linear feet of scratches, you are not far off.
I have a question. A friend of mine has a blue hull (blue sides - grey bottom and deck) boat mid-1980s vintage. Lots of scratches - same situation as you. However, it appears that most have gone into the gelcoat such that they are light blue. I assume that this means that each scratch must be filled individually with a matched color. Is this correct? Would it be better to just repaint the whole hull with gelcoat? Is that possible?
Thread starter #12
My internet connection has been completely hosed for about two weeks now; I've taken 30 or so photos I want to post, showing differences between scratches I filled and scratches I'm just sanding out.

To answer the last question, it sounds as if you're saying there are two layers of gelcoat, with different colors placed one over the other. I find that hard to believe and suspect that the scratches are just oxidized differently, and that is what's causing the color difference you see. But I've been wrong before, and I plan to be wrong again in the near future. ;)

I found that filling the scratches was pretty much a pain.

After sanding the entire boat with 60 grit, there were three classes of scratches:

1. Those that were already gone
2. Those that were deep enough that they needed to be filled
3. Those that were still there, but shallow enough that a little more sanding will take them out completely.

Sometimes, deciding the difference between 2 and 3 takes a judgement call. I made multiple passes around the boat with a pencil, marking the ones I decided to fill, and filled scratches in two separate passes.

Scratches need to be deepened and roughened for filling. I tried using a nail, two different kinds of X-acto knives, a dremel, and an old pair of scissors.

The dremel removed more stuff than I wanted, and I had trouble keeping it IN the scratch. The fatter X-acto knife worked better than the thin one. The tool I liked best was the old pair of scissors. Thick, sturdy, easy to hold onto, dug in to about the right depth, and stayed in the scratch I was following.
Thread starter #13

From 10 feet away, it looks great.

One of these days I'll post the photos. But there are a couple key lessons:

1. 60 grit is too coarse. By the end of the job, I was visibly thinning the gelcoat and had worn through a couple of small "high spots" (such as at the mast step.) There wasn't enough gelcoat left for me to sand out all the sanding marks left from the 60 grit. So my "finished" hull still has thousands of tiny 60 grit sanding swirls. Waxing hid the more superficial ones. You MIGHT be able to successfully use 80, as there's a fair difference between 80 and 60. But I'd consider going coarser than 100 only with great reluctance. But that WILL make the job take longer -- and it already took the better part of three days.

2. Most of the improvement in appearance came from simply using rubbing compound. I'd say I got 80% of the improvement from the last 20% of the work -- which was the change from the heavily faded and oxidized hull back to a bright color. Obviously rubbing compound won't remove any but the most superficial of scratches, though.

For someone considering refinishing a hull I'd suggest rubbing out the hull FIRST. If you're happy with the result, great! If not, you're now in a position to contemplate -- are you willing to spend 5x more work than you've already invested, getting to the next phase?