trying to prep the body for painting, and i bought a cheap tesco value flat sander.
would i have been better off with something like this?
eBay Item

i'm finding that even with an 80grit paper, the flat sander is taking ages to key the surface, and when it does there's big scratches, and its still shiney from the gel coat between scratches.
wonder if a da sander will give a better result, with say 120 (or higher) grit?

i've got a big compressor, so air won't be a problem, or if there's an electric one as cheap?

I would go for a DA but I would buy an electric one unless you want to do it wet

Tried a DA sanader on my 3hp compressor and it struggled big time.
A leccy one would be a safer bet.

However, what are you going to do with 80 grit scratches prior to spraying? I reckon 320 would be more appropriate or even finer.

You can also buy matting paste by farecla,
http://www.fareclaonline.co.uk/acatalog/Farecla_Preparation.html
use it with a proper polisher (needn't be expensive) and will be useful later
Silverline Sander Polisher 180mm 1200W | eBay

the cheapest electric one i can find is £75, which is about £80 more than i've got...
i probably will do wet sanding with it.

clifty, i started with 120 paper on the sander i bought, and it wouldn't take the shine off the gel coat at all. in fact, i swear it was getting shinier! thats why i was thinking something with a more agressive action, but finer paper might be better?
i ideally wanted to do it 400 grit paper before spraying filler primer, but since the 120 didn't make a mark, i went for 80 to take the shine off, hoping that would help start it off

i guess air/electric is pretty much a personal choice. air tools you can adjust the pressure to low it down if needed i guess
my compressor is 3hp, 150l tank, 14cfm free air etc, should be up to the job i guess

but regardless of air/electric, a DA sander is the right one to go for then?

[Edited on 14/6/12 by blakep82]

probably, but I don't understand why the gel coat isnt getting flatted. Maybe it is just too coarse (I use 80 grit on rough woodwork, and have used it to shape filler, but never to prepare for paint)
If the gel coat just needs the shine taken off, I really would use that farecla stuff. The da is way more aggressive,

I have an electric Einhall ? DA sander that I brought from Toolstation for peanuts to do just one job which has done loads in the last few years , ISTR it cost about £30 The only reason I brought it was I didn't have a decent compressor at home . Compared to a good air powered sander it is OK

I'm also bemused as to why your sander isn't taking the shine off. What abrasive paper are you using? Is it getting clogged-up?

its not getting clogged up no, but then its no taking anything off. ok its the crap paper sample that came with it, so not the best, but its still a 120 grit paper. i'll try a bit of wet and dry paper in it and see what happens there
the gel coat is very hard, (i've never really paid attention to gel coats before, so don't know if there's different hardnesses?) and the sander is very cheap. maybe the amount it moves (the radius of the vibration?) is very small? smaller than a proper quality sander? maaybe most of the vibration goes through the handle? i don't know. I hate sanders, and sanding...

I picked up a pretty decent DA from wickes, own brand think it was around £35

That said I am rubbing down mine to repaint and using 580 wet & dry on a very firm sponge, its taking a few hours but is giving me a much better finish.

Just do one panel at a time, few hours on back panel, few on nose cone etc, sometimes power is not the best way. Use a spray bottle with water and a couple of drips of fairy to lubricate as you go, a few rubs, rince clean and rub again.

Regards Mark

[Edited on 14/6/12 by mark chandler]

I have a Black and Decker sander, the one that looks like a steam iron. The DA attachment will wallop through gelcoat with ease and you have to be really careful on edges of stuff unless you want to be through to the CSM! The non-DA wibbling triangular base plate also munches through gelcoat and I'm sure any machine that buzzes the pad about should be pretty similar. I've used a cheapo sander at work and it seems to do pretty much the same job.
As you say, try a different paper. FWIW, I use the stuff from Screwfix.

I too am baffles as to why you are having such issues sanding gelcoat. There is a chance the manufacturer of the part put carborundum grit in it... It makes gelcoat/resin virtually impossible to cut or sand.

I bought the Clarke DA electric from machine Mart for abou £25. Bloody marvellous, gets used on everything now (house, boat, car...).Even floorboards.

I have tried a couple of DA sanders on my 3hp 14cfpm compressor and they hardly rotate. I can't get a rattle gun to work either. It's because I piped it up with 10mm bore pipe and should have used a min imum of 13mm, the existing pipework simply can't deliver sufficient air fast enough.
Having said that a working DA sander is the way to go if you have a large amount of area to cover, but if you are only flatting then working by hand with water is probably best.

i wouldn't go for an air driven one. Air is best when the tool is used continuously and an electric equivalent will overheat. I have 2 DA sanders, a Bosch and a Makita. They are used for hours on end and have never suffered from overheating as the vacuum pulls enough cooling air through.

You would need a big compressor which will cost more to run as it is less efficient. Also air tools are much heavier and you will certainly notice the difference. The Makita is lighter than the Bosch and I know which one I prefer after a few hours.

Your problem is almost certainly the qualty of the abrasive. What comes supplied with the tool will be the cheapest rubbish they could source. Buy 240 then 400 grit papers from 3M or Norton and they will cut through the gel easily.