Paint/finish problem


This past summer I brought an older canoe out of the rafters to paddle.
Before I used it I cleaned it up and also gave the paint a quick and pretty gentle rubdown with Penitrol. It really brought the color back up nicely.
At the time I remember thinking that I should give it another coat of paint at some point. The finish was in really good shape but it was showing some wear.
I originally painted the canoe with two coats of Pettit Shipendec Blue directly over the filler. I wet sanded the filler before painting and allowed it to dry for a long time after filling and also before painting.

After several days of paddling the canoe I noticed a couple very small spots where it looked like paint was lifting from the filler. I was able to rub the paint right off with very little effort.
Since then I have looked the canoe over and it looks like there are only a few very small areas where this happened. My current plan is to carefully sand the entire canoe and then repaint it.

Before I re-paint I am interested to hear comments about what happened and what if anything I should do to make sure the paint adheres. Was it foolish to rub on Penitrol?
This is the first time I have done this and the result (a fleeting but nice gloss) may not have been worth the consequences.

I am also curious to know what other paint I might use over the Shipendec since it looks like Royal Blue is no longer available. Would a Kirby paint adhere over the Pettit?

I will appreciate all advice about this.


I do renovation work and we use Penetrol when needed to reduce brush drag, for better spraying of oil based paints or to increase pot time. I have never know anyone to use it straight as you described. It is also suppose to improve adhesion so maybe concentrated it doesn't flash off fast enough and harms dried paint.

There are so many different things that can go wrong with paints and fillers its almost impossible to make a anything but an educated guess and hope for the best! When you start combining different paints by different manufacturers at different ages it all ends up very much like voodoo! :eek: Sticking with one kind of oil base or water base paint is about the best that you can do. Most oil base exteror grade paints should be compatable with each other. Blisters are most often cause by moisture from trapped water vapor. One paint film will pass water vapor and the other will not, the vapor is trapped and the blister forms. The same thing can happen with oil vapors as when the oil base filler has not cured long enough before it is painted.
The best cure for most blistering problems is a hard sanding, a long drying or curing time and new paint that is as compatable with the filler as possible.
What grit did you wet sand to? Most wet sanding is 220 or finer and most enamel manufacturers specify that you sand the surface to a coarser grit than that for proper adhesion. It's quite possible to sand a surface so smooth in an attempt to get a perfectly smooth finish, that enamel won't be able to get an adequate grip on it and will eventually peel. Enamel is thick enough that you can most likely end up with just as nice of a paint job by painting over 100 grit dry sanding as you can over a higher grit wet sanding. The difference is that the 100 grit job will be attached much better.
What grit did you wet sand to?

It's been many years since I painted that canoe so I am not sure. That said, in the areas where the paint came off the filler looks extremely smooth, almost polished. I think that I did sand it too smoothly. I will probably use a courser paper when I repaint it.

Any thoughts about what paint I can use over the Shipendec? Rollin's point about using like paints rings clear.

By the way, I started wondering what possessed me to rub Penetrol on this canoe so I searched the threads on this site and found this discussion where I had first heard about doing this:

I wonder if Kirby is more forgiving?
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100 grit

Todd, your reply on 100 grit is very appropo as we are in the final stages. we have our first coat of filler on. I have never gone finer than 220 but I think I'll try the 100. I generally only use the 220 for the final coat and 150 for the others. My canoe building skills, as a hobbyist are not show worthy. But I always am trying to improve with each project. this adds to my knowledge.

BTW, not to hijack too much, but we used Bill Clements filler this time. Sure do like how it soaks in. I think it'll make a good foundation.
As far as I know, it's a crap-shoot as to what current paint to try now. Knowing that there are areas where the old paint has been coming loose is a little bit scary, since there may be more of them waiting to do the same. The adhesion of any fresh coat of paint is only as good as the adhesion of the weakest layer(s) under it. You also have to look at the subject matter - which doesn't really make for an easy, predictable paint job. You have a structure made from thin wooden pieces, which may expand and contract a bit on their own. To make it worse, their grain runs two different directions. Then it's covered with cotton cloth, which is not particularly dimensionally stable. The cloth is saturated with filler, which brings a whole different and rather narrow set of flexibility/elasticity characteristics to the mix and this is finally topped with some sort of primer and paint. Then we take this device with all it's differing finish layers, put it on top of a car in the sun at 90 degrees in the summer, leave it in an unheated space in winter and get it wet on those days when we actually get to use it. Sounds like a paint manufacturer's nightmare to me.

Most of the commonly used boat enamels will do OK over a properly sanded old paint layer that is in "good condition" (or at least that's what the directions on can will specify). The important part about "good condition" is that the old paint needs to be well attached and not contaminated by leftover surface treatments like wax or silicone polishes. One rather crude (but effective) test for paint adhesion is to go to various spots on the hull, apply a hunk of duct tape about 12" long to the surface, rub it down really well and than pull it off quickly and in a direction perpendicular to the hull's surface. (I told you it was crude :) but you might be surprised at how well it sometimes works.) If you have hidden adhesion problems going on - you will probably find them. Drastic action? Yes, but on the other side of the coin, do you really want to leave spots on the hull where you could peel the paint off with nothing more than a piece of tape?

I suppose it depends on just how deeply you want to get into the project, but I would be really be tempted to fire up the random orbit sander and take most of the questionable old paint off - down to the point where there may be some color left, but I could see the filler through it. Last time I did this, it took the better part of an afternoon and a goodly portion of an $11 box of Home Depot, 100 grit disks to do my 16' Guide. Once done though, I knew I had a solid base to paint over. I finished it off pretty quickly by going over it with 150 grit on the ROS and it was ready to dust off and paint. With most conventional enamels and even some of the really glossy one-part paints like Brightside, the scratches from 100-180 grit (dry) will be hidden by the paint and still be coarse enough to give the surface some tooth.

Two-part paints, like linear polyurethanes are different. First of all, most use pretty strong solvents that can melt and wrinkle old conventional paints if applied over them. Secondly, most LP paints are quite thin, hide fewer flaws and need a smoother surface to give a nice finish. They are very tough paints, but unless you really know what you're doing they probably aren't a good choice.

I haven't had particularly good luck applying water-based paints or varnishes over oil-based, or oil paints over water-based. I just get the feeling that it's a better all-round bet to stick with something similar to the original layers. Shipendec is a traditional alkyd enamel and that's probably the type of paint that I would stick with - whether from Pettit, Interlux, Kirby or something from the hardware store.