Epoxy and sand for fairing.

[GILow]

Originally Posted by Tillsbury

I would definitely go with the filled epoxy plan. Making a void underneath will guarantee that it will fill with rancid water and junk.

Oh so very true. This area in the keel of the boat had a number of failed aluminium tanks when I bought her. The smell from the crud around the tanks was indescribably bad. NEVER going that path again.

[HankOnthewater]

By all means use polyester resin or its vinylester variety. Thicken it with whatever you have on hand, if the job does not need lots of inherent strength.
If you use any resin:
- mix the catalyst, or part A with B first, then add the filler
- do not make the mixture very hot, or if using epoxy: use a slow (tropical) variety, as you are making large batches, and the hardening process is an exothermic reaction, creating lots of heat. In addition to that, do not start on a hot day. Using smaller batches gives you also more control
- work in layers, and apply next layer before the previous one is completely cured
- sand is a filler, talcum powder works very well

With these kind of fillers it is hard to get an even surface , particular in hard to reach places. Maybe you could leave the second last layer quite rough and uneven, and then use a thin or a mix without a filler to create a very smooth top layer, using gravity.

If you want to drain it forward or aft, place large weight on the bow and stern, to make it drain the opposite way (when the boat is level again). A 44 gallon drum filled with water might just work, or on a boat your size, 2 of these.

I have done the above several times, using the cheapest resin with talcum powder, and adding with each layer a chopmat or wowen mat. The hardest part to get good adhesion on a dirty surface, and if you don't, water will seep in between the old surface and the new resin base'. Likely an angle grinder with a very rough sanding disk is called for and clean, clean and clean!

[GILow]

Thanks mate. Great tips from an old (not too old) hand.

[TeddyDiver]

You might want leave a small sump in the surface for easier cleaning. Thou it might require a bit thicker fill to do that. Sand blasting sand is good and dry for filling, some cap-o-sil to do falls towards the sump. Without silica you get more or less level surface.

[thinwater]

I stand corrected. I toured one factory where it was made by burning silicone oil. They were making coatings and perhaps they had a peculiar spec. I was manufacturing other ingredients, mostly glycols. Yeah, I've bought sacks and know it is CabOsil. I have a big sack of M5 now.


The important point is that it is not ground from sand and is non-crystalline. On the other hand, epoxy that has been filled with sand will produce crystalline dust if ground, which is dangerous. Of course, this is true of most concrete cutting, and is too often overlooked. Keeping it really wet is key.

[Franziska]

Cabosil is a thixothropic agent and meant to be added to catalysed resin as to make it less "runny" when applying it to vertical surfaces or overhead.
It's a pain to mix it into resin as it is so very light.
Don't think you need it in your application.

Sawdust, glass microballoons or similar are a much better choice.
They are easily mixed in.
You can also use sand, especially as in your case structural strength and lightweight seem to be unimportant.
Just make sure the sand is dry and not to dirty. First catalyse and mix your resin batch, than add the sand and mix.
If you use epoxy you should have a little more time than if using polyester resin.
Watch out to get the mixing ratio of epoxy and hardener exactly right. Epoxy is unforgiving on this.

Just be aware, if you ever need to dig it out again, sanding epoxy thickened with sand will be tough. You might have a better chance than using a hot air gun & chisel. Use a mask than against any toxic fumes...

When you use thickened epoxy, do not mix to much, pour it and wait with the next layer until most of the exothermic heat has dissipated.
Don't wait too long though or the layers would need to be sanded in between.

[Scout 30]

Fiberglass Coatings Superbond Epoxy is already thickened & is very easy to use. I've used it below the waterline & it holds up really well. Comes in different size kits.

https://www.fgci.com/item/135361/Sup...llon-Kit-Tubs/

[Singularity]

-If a void that communicates with the outside world was created (per my earlier idea), something went wrong.

-Polyester resin costs $5++ per pound. When someone says "saving weight" in a layup I first interpret this as "saving money." Others' mileage may vary.

-It makes no sense to me to use any filler that has absolutely any resin absorbancy in this application as doing so will just end up costing more $resin. The extremes of non-absorbant fillers includes sand that is free on one end, or lead shot costs <$2/pound on the other. (density vs gap filling TBD)

-Otherwise, where odor (ultimately related to biologics) is a concern, one is best served to reduce the surface area of the space to be kept clean. This means a surface that is as smooth as possible. Any granularity to the surface will tremendously add to the surface area (for example, as per a quick google review, the human gut has a surface area of over 300m^2). If a slurry is used, regardless of the filler used, I'd endeavor to lay a plastic sheet over the slurry during cure to leave as smooth a surface as is possible.

[stormalong]

I had a similar problem with internal ballast. My boat has internal and external ballast. It is a shoal draft version. They cut 13 inches off the keel and put the equivalent weight in the sump in the form of bars and lead shot. Then they poured resin over that. The resin soaked in but left lead shot somewhat loose on top.

I mixed up a slurry of microbaloons and epoxy and just painted on a couple of coats. That was in 1991, I still have the same boat and the coating is still good.

[hellosailor]

Sand is usually quartz sand, which is basically the same as ground glass. Bottom line is that it is inert so it makes a good filler IF you are starting with clean "washed" sand, not just beach sand with salt contaminating it.

Playground sand, sandbox sand, masonry sand, all are basically clean washed sand that you can buy in 50/100# bags at any big box hardware store or masonry supply.

[Fore and Aft]

I would use Q-cell as it levels out nicely. Can’t say I have ever used sand as a filler in 30 years of mucking around in boats.......Does it soak up the resin and how does it level itself? To my mind you are going to make a mini moonscape in your bilge using sand. One of the ester resins would work, then you can coat it with flo coat afterwards.
Cheers

[RaymondR]

I used a mixture of boiled linseed oil and cement to fill water traps on my steel boat.

[GILow]

Originally Posted by RaymondR

I used a mixture of boiled linseed oil and cement to fill water traps on my steel boat.

Somehow that sounds much more nautical.

[RaymondR]

Originally Posted by GILow

Somehow that sounds much more nautical.

I used white cement and it set to a hard white substance like limestone. Since it formed a slurry it settled to a flat surface.

[s/v Jedi]

From the weekly YouTube video of that Danish guy I learned about expanding epoxy foam. He’s planning to use it as a core inside his new rudder. May be worth exploring this as an option.