I am about ready to fair and glass the bottom of my boat. I used the invisible butt seam method for joining the bottom panels so I have a seam to fair out and hide. I tried Paul's suggestion of sanding down the panels where the glass and resin would join them but either didn't sand enough or added back to much resin. The bumps are not huge. They are only about 1/32-1/16", but they are noticable and would look bad on a finished hull bottom.
My question is: Should I fair the bottom to hide the bumps, fiberglass then fair again after glassing or glass then fair everything at once?
my OPINION is that you will save yourself a lot of hard work later if you fair the hull first. Unfinished wood is much easier to fair than epoxy and glass.
That said, don't look at the finish on my hull.... :lol:
I with Brint on all counts. I faired out Magnolia and sanded until I turned blue. On a friday morning I painted the bottom, turned out great Sat. I taped the water line and painted the sides. pittled around all day doing it. Sunday I went out to look over my great paint job. The sides looked like hell. I sanded another week until I turned blue again then I sanded some more. Then I desided it was good, well good enough for me. This is why I said on ALL counts. Lesson - Just keep sanding and sooner or later it will come to you. That is good enough :wink: Bud
I was thinking I should fair first then glass and fair again but an article I read, by Welsford, made it sould like he suggested fairing after glasing.
To be fair, (sorry couldn't resist the pun) it is possible he was referring to a boat that was already faired, and that the fairing he was describing in the article was to address issues caused be glassing? The article I am referring to is:
http://www.jwboatdesigns.co.nz/tips/ply/index.htm
You can go crazy trying to get a perfect fair look! Two ways to look at this. One, spend tons of time on it, two, anyone looking that close is being rude!
It is amazing how you think you have it perfect and then the first coat of paint goes on.... oh my. What has worked for me very well in the past is get it as fair as I can while wood, then the cloth (if used), then primer paint. When this shows up all the pinholes etc I missed, I simply spend about an hour going around the hull with something called "Spot or Glazing putty" (automotive store, comes in a tube). This harden quickly, has never been a problem over years of use, and sands easily. Only for small problems, but at this time that should be all that there is. Paint over it and Bob's your uncle.
Yep, fairing can be a chore, especially if it's a first build or you lack experience. A trick to make things easier is to add a slight amount of pigment to your epoxy when you mix up batches of fairing compound, of course different colors for each layer.
I usually use a white base then black, after which start with the colors. This way you can easily see if you sanded too far or need more goo to build up areas. The high spots show up first as you wear through these pretty quick, revealing the color below. Low spots (the fairing bane) remain the last color applied, in spite of sanding you brains out, fill them up.
You can do this with paint too, but I don't bother.
Sand the hull at one angle (stroke in the same direction) from stem to stern, then go back down the same side 90 degrees to it. Look over you progress. Some spots will be obvious, others not until you've painted it shiny. Wet sanding helps you see the lows and highs better if you "catch" the light just right with the eyeball as you look over things while wet.
If you have a friend that is a body man, then elect him to show you the tricks of the trade.
A couple things to consider and a couple to avoid.
Try to avoid sanding the surface of any of the plywood at all before glassing.
The grain in plywood has two characteristics, hard and soft. When you sand it, the soft grain gets taken down quicker than the hard grain. The more you sand the more pronounced the harder grain becomes and the more rippled the surface becomes. End result is a smaller version of a washboard. Now you are going to try to apply fiberglass to this washboard without filling and fairing it out. We add the characteristics of the hills and hollows of the plywood to the weave of the glass and the "flat" surface becomes worse, requiring more filling and fairing and sanding. Add to that a built up seam with glass tape and the amount of fairing and sanding and filling and sanding and fairing and sanding, etc. becomes almost a nightmare.
The problem with sanding down the plywood in an area to recess the glass tape and epoxy to end up flush with the surface is that you end up with a totally uneven surface to work with. And inevitably it is going to be tapered into the deepest portion of the joint and tapered coming out of it on the other side. However the tape isn't tapered or feathered out to the edges. You would be far better off to cut off most of the outer layer of the plywood to a depth a little deeper than the thickness of the glass tape and to just a fraction wider than the tape itself. Then make your butt joint with tape and epoxy thickend with silica. You can level it out to an almost fair state with a plastic applicator and then roll a strip of heavy guage polyethelene over the joint with a laminate roller or a piece of PVC pipe that is wider than the joint so it rides on the plywood on either side of the tape. After it cures, just pull off the poly and you will find what amounts to an absolutely fair joint. Then just a little fairing compound of epoxy and micro ballons and you are probably ready for the glass. And the surrounding surface area is ready too.
Then apply a coat of epoxy over the entire surface in planned applications. Wait until it has kicked off but is still not completely cured, and apply the glass and spread a coat of epoxy over it with the plastic scraper. No dry spots this way. And no epoxy starvation in the weave of the cloth. If a bubble is beneath the cloth, cut it with a razor knive and work the bubble out.
Continue to do this until all of the surface is covered with the cloth. Then before it has completely cured, spread another coat of epoxy to fill the weave of the cloth and let it cure.
This way the surface is a lot more fair than if you sand and fill, etc before you glass it. Follow John Welsford's fairing practices and you can end up with an incredible finish with minimal effort.
Just remember the less you mess with the surface before you glass, the less you have to fair and sand after you glass. Couple light coats of epoxy with microballons and a long board run at 45 degree angles to the length of the surface in both directions will make short work of the sanding and fairing. Anything beyond that unless there are major hollows to fill can be accomplished with high build primer tinted to different colors or shades and built up to fair. Then apply one final coat of primer tinted to approx the final color of the paint and you can see what you are going to have for a finished paint job before you apply the expensive stuff if that is what you are using.
Tinting primer is a whole lot easier and less messy than trying to tint epoxy and microballons or wood flour. And unless things have changed recently a good high build primer is a lot less expensive than epoxy and sands a lot easier too.