Recently, I've been filling quite a lot of exterior timber with marine fibreglass resin + sawdust. But the sanding is difficult. All types of sandpaper I've tried seem to start off ok, but then go blunt quickly, -- even wet-n-dry (the black stuff).

So which type of sandpaper performs and lasts best on this type of challenging task?

What tool are you using to sand with? Grades of paper etc?

I usually use a 6" pneumatic orbital with Velcro pads. Preference for 3M "hookit" discs in 80, 180, 320 grades and hand finish above that.

Foam blocks that take Velcro "hookit" discs are handy to use up some of the better 2nd hand ones of the buzzer also.

This is my preferences when rebuilding Polyester and epoxy resin materials. From fibreglass eskies to full boat restorations.

Hope that helps.

There's 2 different cases:

1) A handrail of curved profile. This I sand by hand using a long narrow block. 60g and 80g paper (haven't got any 40g at the moment). I've tried rhynalox, silicon carbide, and ordinary (Norton) alox. They all seem to become less sharp rather quickly.

2) Some exterior steps. I spread a layer of sawdust-thickened FG over the treads and now must sand it down heavily back to bare timber (since the point was to fill the cracks in an exteriorly durable way). So far for this I've used a Makita belt sander with 25g and 40g belts (Norton), and then a Hitachi 5" ROS with 40g Norton pads. The treads were a bit cupped to start with, so I also wanted to continue heavy sanding until that's corrected, but it seemed like I only get so far and then nothing more happens. It's like the sanders (both belt and ROS) stop cutting and merely "polish". The surface did get rather hot before I realized it, and I'm wondering whether this is a factor?
Is this a timber-hardening thing? Or simply melting/softening the pad surface? The ROS was on maximum speed, so now I'm wondering if maybe slower is better?

In both cases, I don't need to go to high grit. My problem is really about taking off the excess FG and correcting the cupping. After that, sanding up to 120g would be enough for my needs.

Cheers.

Hi strangerep.
Firstly, I hope you are not using any type of polyester resin in your work on timber.
It really does not have a good adhesive quality for timber work.

80 grit is the harshest grade you would ever need to use, if you are not reshaping an entire piece of work.

It is important with your resins to clean the surface before sanding. Acetone removes any waxes which form on the surface of resins and that wax will clog your paper, no matter what it is.
Also do not work, "too hard". Putting a lot of effort/pressure on you equipment or hand sanding, does not help your paper cut.

For epoxy resin, the longer you leave the resin to cure the better result you will achieve, about 5 to 7 days to let it cure. This leaves you with a very hard finish, which means you can work with finer grit.

I have a lot of grinders, sanders and orbital sander. With only one exception, do I have any grit lower than 80. Most are 120 to 240. Plus some finer grit for orbital finishing.
I always hand finish to follow the timber grain.

Hope this helps. Your question is a pretty big one and I hope more forum members can add more.

Paul.

__________________
I FISH THEREFORE I AM.

Not any more...

Yeah, I found that out the hard way: I filled some holes in recycled hardwood with builder's bog (polyester), and although I painted them thoroughly with many coats of oil-based primer/undercoats and weathershield top coat, the filler started to come loose after a year or two outside. I've recently finished digging it all out and replacing with epoxy FG resin.

I'm surprised. I would have thought a lower grit would be ok for removing the resin that's standing proud of the timber, since the standard technique (or so I believed) is to over-fill a hole or crack, and then sand it down. (?)

Thanks! I didn't know that.

OK -- I know the principle of "let the tool do the work", but I'm never quite sure how much pressure to use when hand-sanding.

...even if I'm trying to entirely remove a thick rough coat so that the epoxy remains only down the cracks?

Cheers. Learning is one my few remaining pleasures in life.

Yes, you are right. Let the tool do the work. Just do it as gently as possible and if it has a dust extraction port, the hook it up to you vacuum cleaner to keep the paper as clean as possible.

If yo really have a "thick rough coat" of epoxy, then you are using too much or do not have your application method worked out.
Some auto body applicators, (plastic) will help you pull the epoxy resin into your work and leave you with a workable finish. This only takes a little bit of practise.

Again I hope this helps.

Paul.

__________________
I FISH THEREFORE I AM.