Sander to flatten gelcoat

[fisherman]

I have always used cheapo orbital sanders with wet and dry, work OK ish but generally the worst problem is fixing the sheet, it usually tears fairly quickly. Thought I might buy a big Makita or some such. I want to flatten down my previously Perfectioned hull, about 30sq metres, trouble is perf is quite hard. Any experience? Small orbital? There's a couple of Makita half sheet sanders, 240 or 110 doesn't matter, same price £200. I have got a biggish belt sander but it doesn't like anything not dead flat.

 

[Bertramdriver]

I have always used cheapo orbital sanders with wet and dry, work OK ish but generally the worst problem is fixing the sheet, it usually tears fairly quickly. Thought I might buy a big Makita or some such. I want to flatten down my previously Perfectioned hull, about 30sq metres, trouble is perf is quite hard. Any experience? Small orbital? There's a couple of Makita half sheet sanders, 240 or 110 doesn't matter, same price £200. I have got a biggish belt sander but it doesn't like anything not dead flat.

I've just invested £7.50 in a 4kw rheostat? Bought on eBay which allows infinite voltage reduction from 220 volts downwards.
Connected to my angle grinder fitted with a 150 mm backing pad £9.95 eBay I now have a slow rotation polisher that is light enough for my arthritic hands to hold. Much better than those two handed Silverline weapons

 

[davidej]

Would a DA (dual action) sander be a better bet?

 

[Bertramdriver]

Would a DA (dual action) sander be a better bet?

Yes at 10x price. Those that don't earn have to use their brains.

 

[PCUK]

A cheap DA sander will be the ideal tool.

 

[Avocet]

Depending on how good a job you want, the thing you really need is a big, curved footprint. Any sander (random orbit, DA, whatever) runs the risk of putting small "flats" on any curved surface if you hold it there for a fraction too long. It's an utterly vile job, but "long boarding" is the way to go for a good finish. Under instruction from Oldsaltoz, I made one out of a 500mm long x 115 wide (or whatever width your sandpaper roll is) x 6mm thick bit of plywood with a handle on each end and a wooden block to clamp the paper. It's thin enough to follow the curve of the hull.

I tried one of these - which was quite good but you struggle to get discs for them and they're now discontinued. Also was a bit "top heavy" and could dig-in if you weren't extremely careful:

http://www.ebay.co.uk/itm/ERBAUER-PSM4009-3D-CONTOUR-SANDER-/281130790112

If you have access to a decent sized compressor, one of these worked best for me:

http://www.justoffbase.co.uk/Air-Sa...-Sealey-SA32?gclid=COXToKnBj7sCFYgKwwodWUUAgA

 

[MYStargazer]

I've just invested £7.50 in a 4kw rheostat? Bought on eBay which allows infinite voltage reduction from 220 volts downwards.
Connected to my angle grinder fitted with a 150 mm backing pad £9.95 eBay I now have a slow rotation polisher that is light enough for my arthritic hands to hold. Much better than those two handed Silverline weapons

This is exactly what I need. Is this easy to do (for a numpty)? How did you do it? Do you have a link to the eBay vendor?

 

[VicS]

I've just invested £7.50 in a 4kw rheostat? Bought on eBay which allows infinite voltage reduction from 220 volts downwards.
Connected to my angle grinder fitted with a 150 mm backing pad £9.95 eBay I now have a slow rotation polisher that is light enough for my arthritic hands to hold. Much better than those two handed Silverline weapons

This is presumably a 115mm grinder not one of the 230mm/9" ones.

Good idea. In fact reminded me that I had an electronic speed controller of suitable rating . Works well with my compact 115mm grinder. Might try it with a polishing head on!

 

[lw395]

I have always used cheapo orbital sanders with wet and dry, work OK ish but generally the worst problem is fixing the sheet, it usually tears fairly quickly. Thought I might buy a big Makita or some such. I want to flatten down my previously Perfectioned hull, about 30sq metres, trouble is perf is quite hard. Any experience? Small orbital? There's a couple of Makita half sheet sanders, 240 or 110 doesn't matter, same price £200. I have got a biggish belt sander but it doesn't like anything not dead flat.

If it really is just taking the gloss and tarnish off, not smoothing out runs or anything, or removing quantity of paint, do it by hand.
Otherwise, random orbit sander, buy cheap sander but the best discs?

My old Bosch orbital rarely tore sheets. FWIW.

 

[fisherman]

do it by hand.

QUOTE]
Wash your mouth out with soap and water! Thanks for the suggestions, like the rheostat idea, as I have two decent angle grinders. I usually use sanding mesh, which lasts longer and doesn't clog, I'm not anticipating a super smooth finish, hull is a bit beaten up. Maybe the dual action is the one.

 

[PCUK]

DA sanders were designed for the automotive industry to follow curves and not to flatten them. Not to be confused with orbital sanders that are designed to flatten surfaces.

 

[lw395]

do it by hand.

QUOTE]
Wash your mouth out with soap and water! Thanks for the suggestions, like the rheostat idea, as I have two decent angle grinders. I usually use sanding mesh, which lasts longer and doesn't clog, I'm not anticipating a super smooth finish, hull is a bit beaten up. Maybe the dual action is the one.

Reminds me of a quote from a builder.
It's a horrible job, we've got a machine to do it. Machine called Kevin.
I'm not sure of the rheostat malarkey, the motors may not like it. You may find they are easy to stall but still go too fast when lightly loaded?
A thyristor controller might be better, like a big dimmer switch.
A proper sander/polisher might be the way? I don't have one, seem to have every other sanding tool though!
They seem darn heavy though.
My screwfix random orbit (the disc spins and its axis is moved around) is pretty good. Not sure it's up for wet work though.

 

[Amulet]

"Random orbit" and "dual action (DA)" the same thing? Seems so to me. Orbit and turn slowly avoiding swirl marks characteristic of orbitals.

By "flatten" do you mean:
- remove local surface irregularities?
- fair to a smooth curve (as some have interpreted)?
- flat a gloss surface back to a keyed surface for a coating?

I'd hate the fairing task, and have no opinions about it apart from trying to make it someone else's problem.

I'd go with random orbit with a soft backing for the other two, with paper no coarser than necessary to the job. My Bosch random orbit came with a firm backing pad, but a soft one was available for curved surfaces - I bought it and use it mostly.

What's with the rheostats? Even modestly priced machines come with speed control which works fine.

I wouldn't take my belt sander anywhere near a task this delicate.

 

[Blueboatman]

The sandpaper clogs less with a 'proper' random DA than flat sanders or cheapo orbitals, and as said, you can 'see' the performance difference and actually feel it as you work the surface and it will be more comfortable quieter and quicker and last " industrially ".
Perhaps rent one ?

 

[fisherman]

It's to flat the surface back for overcoating, perfection, will need a bit of filling of relatively minor scratches. The curves are not very demanding, nothing much more than the average car roof .

 

[PCUK]

Yes, DA and random orbit are the same. Flat orbital sanders are the other type.

 

[Amulet]

And I forgot to say - fer Christ's sake don't use a mains machine wet sanding! People have died that way.

 

[lw395]

And I forgot to say - fer Christ's sake don't use a mains machine wet sanding! People have died that way.

It can be done safely. Some machines are designed for it.
But DO use a circuit breaker in line with the sander.

 

[itchenseadog]

As a professipnal yacht painter I always use a dual action sander. Yes they are expensive to buy but anything else will take twice as long and produce an inferior finish. If you cant afford one, hire one from your local hire shop. I have had my current Bosch one for about 8 years, used most days and still going strong. However the art in getting a good finish is down to the skill of the operator.

 

[cliff]

If you have access to a decent sized compressor, one of these worked best for me:
http://www.justoffbase.co.uk/Air-Sa...-Sealey-SA32?gclid=COXToKnBj7sCFYgKwwodWUUAgA

And I forgot to say - fer Christ's sake don't use a mains machine wet sanding! People have died that way.

That is one reason I like the inline air sanders though I also have air driven DA sanders and a selection of open mesh pads. Great for removing antfouling back to the gel coat without damaging the gel coat - spray of water over the area keeps the dust down and it does not really matter if the sander gets wet - only one exposed bearing which is easy to change if it is left wet and rusts (where is the WD40 when you need it?)