Finishing Question

[John Stevens]

I built an 8 foot pram almost 2 years ago with a lot of help from this board. It's glass over fir. I've noticed as time goes on that the small imperfections seem to come out more. The places where there are screws and nails in the bottom of the hull are becoming more visible. When I first painted the boat it looked perfect.

The boat sits outside all year but not in the water.

Is there a reason for this? Is this normal?

TIA,

John

[mrthethird]

Not that I consider myself an expert or anything....

To my knowledge, a couple of things happen that cause this to occur:

1) Epoxy cures very slowly after the initial "flash". It will cure to the touch, which we commonly call "cured", in a few hours, but in actuality will take several weeks to several months to cure completely. In this time, solvents are leaving the material, and some shrinkage occurs. This is normal;

2) The wood of your boat flexes more than the screws and nails that hold it together. The epoxy remains bonded at the surface, but flexes on the exterior. Since the wood is moving more than the surface of the heads of your screws, a ring will appear around the screw heads. This can also happen to a lesser degree around defects in the veneer of the wood, differences in species of joining woods, and, in my experience, wherever the heck else it wants to...

What to do?

A) Nothing, it adds character

Sand the offending spots, bondo and repaint

C) Sand down the boat to the epoxy, and reglass the whole darn boat.

I would pick A or B if there is no sign of darker dicoloration from water around the offending marks or indentations. If there is dark discoloration, water has breached the fiberglass, and repair needs to be done. This is probably unlikely in the short time you have been using the boat, but worth checking.

I, personally, have had this issue more with polyesters than epoxies, but my experience is somewhat limited.

Get a couple more opinions than mine, but that is my 2 bits worth.

[Barry Pyeatt]

If it is the weave of the glass that is becoming visible, it is referred to as "print through". If it is the screwheads and other fasteners it is a combination of factors most likely.

Doesn't matter much that it is fir or other wood. Epoxy continues to shrink over time as it ages and that allows the weave to print through the thin layer of epoxy over it.

If the screws and other fasteners were not recessed into the material and then capped over, again the epoxy shrinks and the depression forms.

If you are in an area where there are broad humidity and temperature swings (Freeze/thaw cycles too), then that will emphasize the movement and make those imperfections more noticable. They happen to most everything with wood boat construction and are part of the "character of wooden boats. And yes, it happens with polyester resins too.

To prevent this when building, filler coats of epoxy or epoxy and microballons are spread over the entire area to fair it out. Often it will take at least two thin coats to fill completely with some sanding to even each coat out. Then high build primer is applied in several thin coats as well with sanding to fair them out. Generally a long board is recommended for sanding all these coats of epoxy or epoxy/microballon coats as well as the primer coats. Wet/dry sanding will bring it fair with progressively finer grits of paper.

Once it is as fair (even) as you would like or accept then the paint goes on over that.

Even with "peel ply" over the glass this will occur as the epoxy shrinks over time. Building up the surface so that there is more to it is really the only solution regardless of wood or resin type.

It isn't necessary to remove the glass to fix it. Only the surface paint and roughing up the surface so a good mechanical bond can be made with a coat or two of epoxy/microballon filler or high build primer. Then building up the low spots and fairing them into the surrounding areas. Finalize by an overall coat of primer and then a ROS will work well with wet/dry paper. Then two thin coats of paint will give you a nice finish that should give you good performance for several years.

There is a good article published in Duckworks written by John Welsford that covers how he finishes out plywood to achieve a "Professional" finish.

http://duckworksmagazine.com/05/columns/welsford/6/free.htm

Most professional builders follow essentially the same steps with glass over plywood or solid woods. There are individual differences but the basics of all of them generally involve plugging the holes over screwheads and filler coats of epoxy/microballons over it and high build primers using long boards rather than ROS to fair it all to even. Then applying the final finish over a completely primed and faired surface.

Just remember that over time all wood will continue to move and small imperfections will occur regardless. They can best be taken care of when it comes time to repaint.

[Charlie Jones]

nothing to add except one correction. Epoxy DOES not give off solvents as it cures. It crosslinks within itself- there are no solvents to be given off.

Overwise, the print through of weave and screw heads will occur over time due to the continuing cure of the epoxy- just smooth it down again when next you repaint and that should be it.

[John Stevens]

Thanks all! Yes it is showing the weave and also some screw holes (dimples). Good to know it just "happens", I thought maybe I did something wrong when building the boat.

I can live with the marks for now, I'll resurface next spring when I paint.

Thanks to all.

John Stevens

[John Stevens]

When I fill these spots in the future do I need to sand it down to the bare glass or can I just rough up the paint with some 120 grit and fill the dimples?

TIA,

John Stevens

[mrthethird]

It really needs to be sanded down to the fiberglass, then scuffed up, filled, sanded, and painted. If you just scuff and paint, your filler is only bonded to the paint, not the fiberglass.

[Barry Pyeatt]

Always back to at least thru the primer and into the resin. If you go all the way back to the glass and actually into it you break the integrity of the glass fiber. You also loose the resin you used to "fair" out the glass. So then it is more thin coats of resin and fairing out to get it to the point you will apply primer and finish to it again. You may want to use something coarser than 120 grit for the process. More like 60-80 grit to give it some "tooth" for a good mechanical bond.

Unfortunately, if you want an invisible fix, you will have to work on a larger area, not just a dimple and then build up the primer to the point where it is ready for an overall coat of paint. It is virtually impossible to accurately match paint in spot repairs.

[John Stevens]

Thanks,

John

[mrthethird]

Down to the resin, that's what I MEANT....GOSH!!!

[Charlie Jones]

60- 80 grit?? YIKES!! I don't even HAVE those grits in the shop except on the big grinder.

But yes you'll have to sand TO the resin. I would use 100 grit- I feel 60 or 80 grit makes scratches that are too hard to fill afterwards. Butif you are filling with resin you have to get down so the new resin will adhere to resin, not paint. You CAN use some acrylic filler- comes in a tube like toothpaste and fill very shallow dips and scratches by sticking it to the paint, but not for anything large. I use a product made by 3M. I use white for light paint but I also use a red for darker paints. The white is a marine filler, the red an auto body stuff- both acrylic, both work well.

[mrthethird]

West Systems recommends 80 grit sandpaper to create a good mechanical bond. System Three recommends 50-60 grit. There will be little electrical bond, since the base resin is cured. So gouge the surface with 80 grit, fill with "filler of choice", then sand smooth with whichever grit you would like to finish with. Check the finish manufacturer's recommendations on prepping the surface.

[Barry Pyeatt]

OK, OK, Charlie, I'll conceed, 60 grit is a bit agressive for paint removal where print through or small dimples, etc. are concerned. :roll: And on a belt sander or other really agressive power tool perhaps even 80 is too. I did have several 50-60 grit wheels for my grinders. I will also have to point out that most of my repair work was commercial as was most all of the building. So a fine yacht finish wasn't a primary criteria. Getting it done and out of the yard or shop was the primary goal with it holding up to the elements and abuse. We still made every attempt to make it look good though.

Having said that, I still use 80 grit most often to take off paint and primer with a light touch. then go to 100 and/or 120 before I prime. It is easy to dramatically scar up a surface and create a lot more work if you are too agressive with removal. Never had a mechanical bonding failure though.