How to sand between coats of paint?

[dndrich]

SCAMPers:

I primed the hull with Interlux Pre-Kote, and then sanded it back to translucent in some areas as recommended by Interlux on the can. Now, I rolled and tipped Rustoleum Gloss Sail Blue last night. I thinned the paint around 5% with mineral spirits. That worked well. I have included a photo. But, I need to apply at least one more coat, and probably two more coats. How much do I sand between coats? Do I use the orbital sander, or do it by hand? Thoughts here?

IMG_2654.jpg

[Peter E]

You apply another coat if you can see areas where the primer shows through.
Sand lightly, The purpose is to provide tooth for the next coat. I would use 200 hundred or 300 hundred paper.

In the photo it looks good now. In a year from now if you use the boat a lot you will be doing a lot of touch up, so don't use up all your paint.

I think a painted parts of my interior at least twice or three times. It took a beating today a little sand or gravel on your feet is hard on paint.

[PAR]

Finish work (coats) is all about the prep. If your prep was good, the boat fair and smoothed (yep they're different), a light sanding between coats is all that's necessary with most paints. 220 is the usual recommendation, preferably wet (keeps down dust, contamination, produces a better result, etc.). Some like to take it to 280, but for the average backyard build, the prep, fairness and smoothness of the project can make this hard to justify. As a rule, when you sand top coats, you have to consider you'll remove at least half the film thickness of the previous coat, if not more in some places. Film thickness is key to a good paint job, not too much, not too little. A minimum of 2 full coats on a roll and tip job, with 3 not being incommon, after sanding and finding "issues", that need fixing.

The top coat you're using is one I'm very familiar with and it's pretty much a straight up alkyd, so a 5% cut will help it flow a bit, but Penetrol will do a better job of permitting it to "lay down" eliminating brush marks. That particular Rusto isn't as full bodied as others, so stick with the 5% spirits cut.

Now all this said, if you've put lots of time into fairing and smoothing, you can consider 280 maybe up to 360 (wet of course) to really bring out the finish, when 'tween coat sanding. 360 is really pushing it on a roll and tip job, as you're getting into the polish range of grit sizes and adhesion issues can crop up. I routinely use 360 on top coats, but also follow up with cutters (compounds) and polishing too, on a fully sprayed paint job, so a different animal.

What is this fair and smooth business? Fair is what you can see and smooth is what you can feel. For example look at a car door with a big 'ol dent in it. As soon as the light washes over the dent, it's obvious to all, so not very fair, but if the car was buffed up recently, it may very well still be quite smooth. Fairing operations are first in the prep. You'll knock down the high spots, any waviness around seams, tape, fastener heads and fill in the lows to surrounding areas. This is the putty stage and filling fabric weave, etc. You'll come to hate the "board of pain" (the longboard) and you're elbows usually are the biggest determining factor, in how fair you actually get the boat. Assuming you're young or just into pain, smoothing operations start. This is simply taking the surface from medium grits to finer grits. You might still use the longboard (I do initially), but eventually you'll be working the DA sander with 100 - 120 grit on the primer coats. During smoothing, you'll need to "ghost" the surface with differing colors of primer. There's special paints for this or you can use printer toner and even a very light dusting of rattle can primer, of a contrasting color (auto body guys call it a "guide coat"). The contrast helps you see, extremely subtle variances in the undulations on the surface (amazingly enough, you can actually see a 002" variance in the surface of the paint). The Rusto you're using doesn't need anything more than 120 grit on the primer, being full bodied enough to easily fill these scratches, in a single coat.

Lastly, be honest with yourself about the prep. Was it fair and smooth or just kind of fair and sort of smooth? The reason is you have to be careful what you wish for, in regard to the top coat results. A nice glossy, smoothly applied paint job will show up every single defect and blemish in the finish. Many choose semi gloss and satin, instead of gloss for this reason, both of which tend to hide minor imperfections. I've seen countless backyard builders, spend big bucks on fancy two part polyurethanes, just to find it looks like a quilted mattress in the sunshine, because it was reflecting all the suicidal bugs, dust and other imperfections in the freshly applied gloss.

[dndrich]

Finish work (coats) is all about the prep. If your prep was good, the boat fair and smoothed (yep they're different), a light sanding between coats is all that's necessary with most paints. 220 is the usual recommendation, preferably wet (keeps down dust, contamination, produces a better result, etc.). Some like to take it to 280, but for the average backyard build, the prep, fairness and smoothness of the project can make this hard to justify. As a rule, when you sand top coats, you have to consider you'll remove at least half the film thickness of the previous coat, if not more in some places. Film thickness is key to a good paint job, not too much, not too little. A minimum of 2 full coats on a roll and tip job, with 3 not being incommon, after sanding and finding "issues", that need fixing.

The top coat you're using is one I'm very familiar with and it's pretty much a straight up alkyd, so a 5% cut will help it flow a bit, but Penetrol will do a better job of permitting it to "lay down" eliminating brush marks. That particular Rusto isn't as full bodied as others, so stick with the 5% spirits cut.

Now all this said, if you've put lots of time into fairing and smoothing, you can consider 280 maybe up to 360 (wet of course) to really bring out the finish, when 'tween coat sanding. 360 is really pushing it on a roll and tip job, as you're getting into the polish range of grit sizes and adhesion issues can crop up. I routinely use 360 on top coats, but also follow up with cutters (compounds) and polishing too, on a fully sprayed paint job, so a different animal.

What is this fair and smooth business? Fair is what you can see and smooth is what you can feel. For example look at a car door with a big 'ol dent in it. As soon as the light washes over the dent, it's obvious to all, so not very fair, but if the car was buffed up recently, it may very well still be quite smooth. Fairing operations are first in the prep. You'll knock down the high spots, any waviness around seams, tape, fastener heads and fill in the lows to surrounding areas. This is the putty stage and filling fabric weave, etc. You'll come to hate the "board of pain" (the longboard) and you're elbows usually are the biggest determining factor, in how fair you actually get the boat. Assuming you're young or just into pain, smoothing operations start. This is simply taking the surface from medium grits to finer grits. You might still use the longboard (I do initially), but eventually you'll be working the DA sander with 100 - 120 grit on the primer coats. During smoothing, you'll need to "ghost" the surface with differing colors of primer. There's special paints for this or you can use printer toner and even a very light dusting of rattle can primer, of a contrasting color (auto body guys call it a "guide coat"). The contrast helps you see, extremely subtle variances in the undulations on the surface (amazingly enough, you can actually see a 002" variance in the surface of the paint). The Rusto you're using doesn't need anything more than 120 grit on the primer, being full bodied enough to easily fill these scratches, in a single coat.

Lastly, be honest with yourself about the prep. Was it fair and smooth or just kind of fair and sort of smooth? The reason is you have to be careful what you wish for, in regard to the top coat results. A nice glossy, smoothly applied paint job will show up every single defect and blemish in the finish. Many choose semi gloss and satin, instead of gloss for this reason, both of which tend to hide minor imperfections. I've seen countless backyard builders, spend big bucks on fancy two part polyurethanes, just to find it looks like a quilted mattress in the sunshine, because it was reflecting all the suicidal bugs, dust and other imperfections in the freshly applied gloss.

Fun post! Thanks for the good advice. As a novice builder I can tell you that my boat is sort of fair and sort of smooth. Probably better than a 2nd grader, but not by a bunch. And, I like the color even though it is gloss and will show all the mistakes! Oh, well. I plan to beat the heck out of it sailing anyway.

I found about 5% mineral spirits worked well for my first coat. It actually looks pretty darn good given my skill set. True, all of my fiberglass seams are obvious despite my attempts to fair them with 410 etc. Live and learn. If I build another I know what to do. So, for now, I'll wet sand with 180-220, and put on 2 more coats. That should do it.

[Brent65]

Daniel,

You can be very proud of the finish that I see in your attached photo. I wish my boat looked this good when I moved on to the next step. Great job.

[dndrich]

Daniel,

You can be very proud of the finish that I see in your attached photo. I wish my boat looked this good when I moved on to the next step. Great job.

Thanks, buddy! I am actually quite happy with it. And, just like sailing, the building has been a journey. I truly enjoy it despite the fact that I learn as I go. I try not to take it too seriously, and in the end, I will have a very sturdy craft that sails great, and will look cute too. But I figure I can get good advice from the folks building, and make it as nice as I can with the time I have to do it. I sure hope I can splash by late fall!! Lots to do before then though.

[dndrich]

OK, so I wet sanded with 220 to dull the gloss. I did it by hand folding the paper. Worked well. It is really easy to take off paint, so I went for dull only. Had to learn to be careful there, especially at the lap edges. The first photo shows the boat after the dulling of the paint. I then recoated my 2nd coat. It was a warm day today and I was working by myself, so rolling and tipping had to be quick! I still have some brush marks, but it looks pretty good really. I'll probably put on a 3rd coat and be done with that. I am glad I chose this paint. It is inexpensive, rolls well, and seems to get pretty hard once it cures for a few days. It is also easily available.

IMG_2666 (1).jpg

IMG_2668 (1).jpg

IMG_2669 (1).jpg

[PAR]

This is the last lapstrake I built (couple of years ago).

49-sm.jpg

At home on the owner's dock (okay it's a lift).

[dndrich]

That is gorgeous!! Well done.

[PAR]

Thanks, but this ain't my first rodeo . . .