Fairing a hull

[tuantom]

Hi. I've flipped my 24' (7.7m) boat upside down to fix a few persistent old holes and refinish the bottom. It has a bit of a hook; about 3/8" (~ 1cm) in the last 5 or 6' (1.6 - 2m) from the transom where the trailer bunks held it (the keel and chines are still pretty straight) that I'd like to fair out while it's belly-up. My question is: what's the best method for this?

I have epoxy and a couple of tubs of micro balloons and silica, which I plan on mixing 3:1 mb to silica for my fairing compound. Do I use this in combination with cloth? Or do I just fair it with my mixture?
- Tom

[tuantom]

Seems to be a tough one.

Maybe I'll fair about half the total amount, put some glass over that, then fair the rest.

What kind of surface prep is necessary on the existing hull?

[TeddyDiver]

Or maybe just mixing some fibers in the compound..

[tuantom]

Thanks for that suggestion Teddy, it hadn't occurred to me; but it makes sense for holding it all together.
One more question: Does my epoxy (US Composites supplied) become more or less flexible with more additives (microballoons, silica, fibers)?

[PAR]

Wet the area out with unthickened epoxy, let this get to the just past tacky stage then apply a thickened mixture of micro balloons and cab-o-sel or other structural reinforcement. Put this on with a notched trowel. When it has cured, sand with a long board at a 45 degree angle to the grooves, continuously checking for fairness. If you have low spots remaining, fill the grooves and apply another layer of thickened goo with the notched trowel again. When the whole area is fair, fill the grooves with a squeegee or putty knife. Apply two additional coats of unthickened epoxy over the repaired area and prep for paint.

If the hook is particularly deep, you may wan to "bulk" up the area with fabric first, then fair with the notched trowel trick. I don't like using ballons below the LWL, but many do it.

[tuantom]

Thanks Par

[eponodyne]

I'd bond on a couple pieces of 1/4" AND 1/8" Ply and get them reasonably fair first. Wood is a heck of a lot cheaper than Caob-o-sil. Glass over it when I was done, finish-fair, and paint.

[marshmat]

Not sure I like the idea of using 1/8" ply as a below-waterline fairing material, but to each his own....

Colloidal silica is a good strong filler, but it takes forever to sand. I wouldn't object to microballoons if there's going to be a good paint system overtop, though. PAR knows his stuff and what he described is more or less what I'd expect a professional shop to do if you gave the problem to them. If you're patient, you can do it quite well yourself, but you will find that there's a reason why fairing is an expensive job to have done by a pro- it is tedious, exacting work.

When you're done, I would recommend rebuilding your trailer bunks to extend just a little bit past the transom, so you don't get the same thing again in a few years.

[PAR]

Also, reposition the trailer bunks so they land directly under the longitudinal stringers. This helps tremendously in preventing unwanted hooks from appearing.

My filler recommendation is based on the likelihood of this being a powerboat and the area affected is it's plane patch, where considerable slamming load as well as other potential for movement might occur.

[marshmat]

Agreed, PAR... colloidal silica would hold up better under repeated slamming than would a microballoon filler. Depends on the epoxy of course; I haven't used the US Composites product and so can't testify to how well it holds up under flexure and fatigue.

[tuantom]

Yes, the area is the plane patch. Would a 2:1 mb to silica be better? When I talked to US composites, the rep told me their fairing mix was just a 3:1 mixture; but we didn't talk about repeated slamming - just if it was suitable below the water line or not.

Adjusting the trailer is definitely on the agenda.

[ediestel]

Use West systems epoxy.

To fill, use microballoons and silica/carbo; I actually use 80 %/20 %. In case you are looking for more strength you reduce the amount of microballoons and increase the silica or even add milled glass fibers, the last is the strongest.

Microballoons are easy to sand. Thus the lighter you make your patty, the easier it is to sand, the stronger and heavier, the more time it takes to sand.

80/20 for mere filling is good though.

After you have faired the spots, you can lay up 2 ounce fiber and wet it with epoxy. When it has settled, sand it down just to the fiber.

After that you use a primer, the prefered one is Duratec, and then paint or gelcoat.

Very imporatant: you have to use a spray gun to put on the Duratec primer and the paint/gelcoat layers. This will fill the micropores and your repair will not leak.
It sounds a little exaggerated, but the repair done with a spray gun definitely holds the water better.


Many people would not do the 2 ounce fiber layer, but it is a very good way to finish a very solid job.

[tuantom]

Ediestel, thanks for the suggestion. I do have enough 4 oz cloth left over from the sole replacement - it might be a little overkill; but I already have it. Since I won't have access to any spray equipment or facilities (it's upside down @ a boat yard right now), my plan is to roll on a couple of coats of epoxy, prime then paint.

[marshmat]

As long as you stick with the manufacturer's recommended primer and pre-cleaning procedures for your chosen paint system, you shouldn't have a problem there. But follow their directions to the letter; those directions are what they are because the paint maker has seen it screwed up every which way already and is trying to make sure you get it right the first time so you don't come complaining back to him that the paint sucks.

[ediestel]

I agree that it should work without a spray gun.

I used to do it that way, letting gravity do the job of filling pores.

A very experienced local boat builder always smiled at me when I complained about my fairing, or leaks that I could not explain.
He just said " you need to use a paint gun "

Finally I bought one and the compressor, about $ 250 total - and he is right.

The crucial step for my repairs now is the spray coat containing primer with 10 % acetone.

That layer comes after your basic fairing is done.

That local builder finishes with 2 ounce cloth on all his repairs including gelcoat repairs without exception. Then he sprays, then he sands again, sprays again if needed and does his final sanding before he gets to the final coat.
It makes the area larger that you need to cover with the final coat, and therefor creates another sanding job, but the results are grade A.


I have no good explanation as to why, but the spray gun does fill the pores better and these repairs will not leak, while the gravity/brush related methods did not work as reliable for me.

In case you decide to buy a spray gun, buy one with a 2 mil nozzle and make sure the paint container works with gravity - you will see what I mean when you look at the models. With those you can see how much primer/paint you have left.

It is real fun to use those spray guns by the way, easier to choice grade finish.


In case you intend to use the 4 ounce as your final layer of your repair, I suggest that you don't do it and buy a piece of 2 ounce, unless you need 4 ounce for structural concerns.


The 4 ounce will cause you much more work to finish just because the texture is coarser. With 2 ounce you begin to see your final result the moment you wet the cloth, less extra layers are needed.
You tape the area and let the 2 ounce cloth extend unto the tape. Then you can judge well how deep you can go while sanding, being slightly more agressive sanding over the tape. When the 2 ounce is gone where it had overlapped, it is time to stop. Remove the tape and spray.


My point with the spray gun: if you do repairs more often than once and you want to do it effectively, you might consider the approach that I learned from our local builder.