It's almost a religious belief that if you apply multiple coats of polyurethane finish to wood, you must sand between coats to provide "tooth" for the next layer to grab on to. The reality is that this is not necessary, and the only reason to sand is to remove blemishes, dust etc. No one had ever seen polurethane delaminate! It's the same belief we hold about epoxy. Unless the goo is still "green" you must sand to provide "tooth" for subsequent layers. Has anyone ever seen a coat of epoxy delaminate from the ones under it because it wasn't sanded?
 When I apply epoxy I use a plastic tray and a wide spatula. Between jobs, a thin coat of hardened epoxy is left, and after a while, multiple coats build up. I then "snap" them free of the plastic and am left with a sheet of hardened epoxy, often 2mm thick. Try as I might (I've even used a razor blade!) I cannot get any of the epoxy to delaminate! I certainly didn't sand between any of the coats. What gives? Why sand?
Dave
The only thing I can think of is that West System says their epoxy reaches about 90% strength initially and then cures the rest of the way over the next several days. Maybe during this time it is still able to bond to a new layer.
I think a lot of the precautions and techniques are meant to be real "belt and suspenders" procedures to make sure you get the best results possible. I forget what the time frame is but I do know that for polyurethane paints and I think epoxy there is a time frame where it's OK to do another coat without sanding, up to a full day if not two, if I recall correctly from using LPU to paint my boat.
I know what you mean about the build up on a scrapper build up. I suspect it can delaminate though. I heard load popping noises twice while building my boat and think they were either plywood or epoxy delaminating but since it looks the same on the surface only sanding down through would give a definite answer and I am not about to do that. The noises occured on flat deck areas that were bent/curved after being encapsulated. The deck has 2 coats of epoxy on the top and two on the underside along with fiberglass cloth on the top side and another 2 coats. The strange thing was it didn't happend when I bent/curved the surfaces. It happened days later. I also suspect temperature change might have had something to do with it. I am no expert on epoxy so will wait until one of them on this forum comments but I could not come up with any other explanation for the load popping noises. No broken wood, no seems separated, noise came from the bent/curved area and no place else. I was standing by it when it happened.
It depends on the polyurethane (yes, I've seen it delaminate on several occasions) as to the sanding requirement. From a chemical stand point, the need for sanding is dependent on the type of cure. Most polyurethanes (single part) have no acetate accelerates, so you need to sand these, because subsequent coats need a mechanical bond to remain stuck. On the other hand two part polyurethanes (LPU's) typically use cyanoacetate curing agents, which can be "reactivated" with more of the same, within a usually short window of it's cure process. This means you can over coat while the polyurethane is dry to the touch, but still in the "green" stage of the cure. It will say "no sanding" on the can. Unless you have a good grasp on the chemical nature of the product you're using, don't assume you can just over coat without sanding. As a rule, single part polyurethanes need to be sanded.
Epoxy can be over coated, as most know, without sanding for the same reasons the LPU's can be, a still chemically active process is on going and you can tie into this with subsequent coats if within the activity window. In fact, this is the goal of laminators, to setup their work to take advantage of these windows of opportunity.
All this said, epoxy and polyurethane over coats are dependent on surface prep. I've seen both fail as a result of poor prep. Most of the time (90%) it's this poor prep, not the chemical combination of goo's that cause delamination. Epoxy and polyurethane formulators publish a window of chemical activity, in which you can over coat, sanding free. What often isn't published are the things that can shorten this time period. Humidity and temperature can have huge and dramatic effects on the window, so if in doubt about it, wait until it cured enough to sand and tooth it up. This is especially true with polyurethane clear coats, where the window is usually short.
As an example I use Bristol Finish, which is a cyanoacetate cure, two part (LPU) polyurethane, clear finish. It's exceptionally durable, tough as nails and costly stuff. Once you get a feel for how to use it, it's a nice product, but the learning curve can be daunting. You can spray or brush it and it has about the best gloss retention available. The window for no sanding over coating is about an hour in normal , non-controlled, summer time conditions (finishing in a 90 degree garage). If you have a climate controlled booth, you can extend this to about 2 hours, because the humidity and temperature are much lower. If you are 45 minutes into you window and start your next coating sequence, but the by the time you get to the other side of the boat it's past an hour, the coating will "lift". You can feel it with the brush when you get to the areas that are "hard", compared to the still chemically active areas. The brush feels "slimy" which is the only way to describe it. I've learned this lesson the hard way and removing LPU polyurethanes is a difficult thing with out damaging the substrate.
In short, surface prep and knowing what you're working with is the only way to get away with no sanding between coats. Wives tails and speculation aren't the wisest choices available. If in doubt, let it cure, then sand it.
When I did my multiple coats, on the deck and hull surfaces, I set an alarm and went out every 6 hours until I was finished. At the time that period provided me with about 2 hours to apply the next coat and still be in the green. It took about 45 minutes per coat. It wasn't fun getting up in the middle of the night but I felt it was workth it. I would do things a bit diiferent on a second build and relay on more fillers for a bondoing effect but being the first I accept it as a learning process, well that and Paul hadn't posted his technique for getting a great finish post yet. After that post I knew where I could improve and will, if health holds and I decide to be a builder instead of a sailer, do things a bit different on number two. Sorry but I don't have a link to the post I refer to.
When I did my epoxy and then my two part polyurethane paint, I was too stupid to know about the "window" Paul is talking about, so I waited for every coat to cure, then sanded it, and applied the next coat. It took me weeks. Finish turned out nice but I swore up and down I would never build anything again. I've gotten over it but I still get a rash when I touch sandpaper.
I have well over 100 hours sanding time in my little boat. A bit obsessive but as my wife will tell you I was building a piano not a boat. I suspect your first one is probably always over built. I don't have a love of sanding but I do love working wood. I enjoy the smell feel and look of it as it changes from roungh to smooth.
I didn't really embrace sanding until I made a set of oars. The oars changed from rough square shaped to smooth round oars with about 2 hours of sanding on each. I started with 40 grit in a belt sander to get them roughly octogonal, then 80 grit in a palm sander to get them round and finely 120 grit paper in hand to get them smooth and feather the tapered blades a bit. I just masked up, put on some music and went for it. I actually enjoyed it after a bit and decided it was part of the zen of building a boat. I sand therefore I am kind of thing.
It reminded me of Army scout training. To pass part of the two week challenge test I had to dig in on a beach, sit in the hide hole and count waves for 24 hours. The thinking behind it is you might be assigned to sneak up, dig in and watch a camp, group or individual for weeks gathering information or waiting for the right person to show. If you got the wave count wrong you failed. It was hot, wet, the insects were hungry and it was boring. There were 20 of us at the start of the 2 week challenge. Five of us made it to the beach and only two of us passed the wave count. It taught me that you could space out and still stay focused. That sounds strange but if you think about it we all do it. If you run you repeat a physical motion while completing multi task behavoirs like avoiding traffic, talking to other runners, navigating terrain. If you run long distances solo, away from traffic, you still run but are able to space out the other behavoirs.
Actually somewhat like rowing. The wind stops blowing at exactly eight o'clock every evening here and I am often stuck rowing home. The first mile sucks but after a while some ganglionic part of the brain takes over and you just keep doing it.
You would think that by now, you'd be slipping up to the dock at 7:45 Keith . . .
Paul, for some it is running, some rowing, some sanding and some building just one more boat. Now how many boats was it you have built?Â
