Fairing Upside Down!

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robbieg

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I have some areas on the underside of my hull to fair over the coming months. In a few areas I'll be working overhead-ie on the underside of the hull.

One area is quite large (a foot or so square) and shallow 2/3mm deep. The fairing material will go over an expoxy coating and obviously needs to be suitable for underwater application and preferrably easy to sand.

Any tips on what to use how to approach this so that the material a laboriously fill the area with doesn't drop straight back on my head!

Thanks.
 
From the West System web site:

Adhesive Fillers vs. Fairing Fillers

Fillers are used to thicken the basic resin/hardener mixture for specific applications. Each filler possesses a unique set of physical characteristics, but they can be generally categorized as either Adhesive (high-density) or Fairing (low-density).
Adhesive filler mixtures cure to a strong, hard-to-sand plastic useful in structural applications like bonding, filleting and hardware bonding.

Fairing filler mixtures cure to light, easily sandable material that is generally used for cosmetic or surface applications like shaping, filling or fairing.

ADHESIVE FILLERS

403 Microfibers
403 Microfibers, a fine fiber blend, is used as a thickening additive with resin/hardener to create a multi-purpose adhesive, especially for bonding wood. Epoxy thickened with microfibers has good gap-filling qualities while retaining excellent wetting/penetrating capability. Color: off-white.

404 High-Density Filler
404 High-Density filler is a thickening additive developed for maximum physical properties in hardware bonding where high-cyclic loads are anticipated. It can also be used for filleting and gap filling where maximum strength is necessary. Color: off-white.

405 Filleting Blend
This strong, wood-toned filler is good for use in glue joints and fillets on naturally finished wood. It mixes easily with epoxy and lets you create fillets that are smooth and require little sanding. Its color is a consistent brown, so 405 can be used to modify the shade of other WEST SYSTEM fillers.

406 Colloidal Silica
406 Colloidal Silica is a thickening additive used to control the viscosity of the epoxy and prevent epoxy runoff in vertical and overhead joints. 406 is a very strong filler that creates a smooth mixture, ideal for general bonding and filleting. It is also our most versatile filler. Often used in combination with other fillers, it can be used to improve the improve strength, abrasion resistance, and consistency of fairing compounds, resulting in a tougher, smoother surface. Color: off-white.

FAIRING FILLERS

407 Low-Density Filler
407 Low-Density filler is a blended microballoon-based filler used to make fairing putties that are easy to sand or carve. Reasonably strong on a strength-to-weight basis. Cures to a dark red/brown color.

410 MicrolightTM
410 MicrolightTM is the ideal low-density filler for creating a light, easily-worked fairing compound especially suited for fairing large areas. Microlight mixes with greater ease than 407 Low-Density filler or microballoons and is approximately 30% easier to sand. It feathers to a fine edge and is also more economical for large fairing jobs. Not recommended under dark paint or other surfaces subject to high temperatures. Cures to a tan color.

They seem to recommend 410 microlight for easiest fairing/sanding.
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I have faired my boat from top to bottom using Sp systems 106 mixed with microspheres and colloidal silica to their recipe. Although most of the work was using gravity to hold the mix to the boat, there were a few areas that were tricky and I just used a plasterers trowel and hawk (I think thats how it is spelt). If I had stickyness problems I made the mix wit a bit more resin to make it gooey. But that does make it a bit harder to sand off. You can download instruction sheets from Sp Systems website.
Wear a bin liner over your head!!!
 
I will have have a similar job once I've filled a 4" hole left from the removal of a skin fitting.

If it wasn't for the underwater bit, I'd use plastic padding or whatever cheap car body filler I could find, but I hae' me doots about polyester filler's resistance to constant immersion, and I've a vague rcollection of having read that it doesn't stick to epoxy too well,

I expect to use the same epoxy resin that I'll use to layup the patch, but mixed with microballons - as thick as I can get away with, 'cos epoxy's hard to sand, but the microballons are (relatively) easy. The official thickness is "to the consistency of peanut butter"
 
Epoxy doesn't stick to the shiny side of parcel tape. If you make a former the shape of the hull where the hole is and stick parcel tape to the face, you should be able to load the hole with the repair paste and press the former against it. Or if possible apply the former first and load the hole with the repair paste from the inside. That should give you a pretty good approximation of the hull profile requiring very little filling or fairing.

You could also retain the repair paste on the iside face of the hull the same way as the epoxy repair paste always seems to run away as it cures.

remember to kind of countersink the hole surface from both sides so the repair paste makes a sort of "diablo" shaped plug that can't fall in or out.
 
Hemple Profill, not cheap but fills well, sticks effectively to epoxy and is easy to sand. Seems very expensive but an £80 tub (two tubs actually about 5 liters in total) goes a long way. Worth the money and I'm tight!

Yoda
 
Some general tips I got from guys that supervised construction/fairing of Chay Blyth's Global Challenge boats (were substantially faired overall, as steel construction - looked fantastic):

Use large notched/comb trowel spreaders. These leave a grroved finsih that is quicker to sand. Grooves will be sanded away quicker at highspots, untouched grooves indicating where to add extra filler on next coat.Sand using long boards (~metre long) and hold parallel to curve axis - like fore and aft for most of hull/bilge area.
 
Hoplefully Oldsaltoz will repply to this thread as he's done rather a lot of this! he's currently talking me through the process on my boat.

To fill, as has been suggested, use an epoxy filled with glass microspheres as it's better under water. Paint some neat epoxy on to the surface first as thinly as possible (I rub it on with a bit of old towel) to help wet it and ensure good adhesion between the filler mixture and the hull.

The technique for applying it that I'm using at present is to mix up a paste and use the back of an old wood saw blade with the teeth either ground off or taped over. Put a dollop of mixture on the edge of the saw, bend the saw round the curve of the hull at an angle of about 30 degrees to the hull surface and drag it along the hull. I'd never have believed it works as well as it does!

When dry, it's worth making a flexible thin plywood board with sandpaper over it. You can buy rolls of sandpaper so just make the board about 18" - 2' long and as wide as the roll.
 
G'day Lostinfrance and Robbieg,

I don't know, a bloke spends half his life working out the best methods to to get a great finish and someone (Avocet) gives it away, good on yer Ian.

Only kidding. The old straight back saw would have to be the best tool in the box, saves hours of sanding and provides a enough stiffness to push the filler in good and hard.

The prime reason for precoating is two fold, if left for a short time the epoxy resin will get tacky and help prevent a slump or plop, the other reason is that the wetted surface will help maintain the level of resin in the mix being applied; it get can get too dry and will not stick or set properly.

Make sure the filler used is a "Closed Cell" type, and consists of balloons or spheres and sanding will be a lot easier and faster.

A long board, more aptly named a torture board can be made with simple strip of 3 or 4 ply material about 15 mm narrower than the strip of paper/grit you plan to use. Add a small block of timber close to each end and fix it with a couple of countersunk screws that will pass through the paper and ply then into the blocks, the countersink is to ensure you don't scratch your nice new finish.

It is also very important to use the correct sanding action, a figure of 8 is the best; imagine a large number 8 on it's side about 2 feet long, as you complete the first 8, extend the last push (or pull) so that the next 8 will overlap the first by twice the width of the board, the longer and wider the board, the harder it is to work, but the better the finish, I find a 3 footer about right for me.

Also remember that epoxy leaves a residue after curing and must be removed before you do any more sanding, just run a garden hose over the area and rub it with a plastic kitchen scourer, it only takes a minute and you are all done when water no longer forms beads.

If you are filling a wide depression you should mix some filler and apply it narrow strips about 3 or 4 mm high, you can use a large syringe or a small trowel with a notch cut out, form the strips up in a hatched diamond pattern with the longer part of the diamond running fore and aft. Then use the board to sand it till some areas around the outer are all but flush, then add more filler between the small lines that are left.

Amazing, it's so simple to do but so difficult to explain; I must get my blog up and running with a stack or pictures one day soon.

Well I hope this helps, if it's a bit confusing let me know and I try again, meantime---

Avagoodweekend angoodchrissie......

0250 hrs, time for sleepzzzzzzzzzz
 
Long boarding, like digging holes in London clay or knocking up concrete by hand, is one of those jobs that it is well worth paying someone else to do!
 
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