Sanding Polyurethane

[Ledzendrix1128]

i got some sags from my poly urethane on the swirl job i did. so i need to save it... i sanded nitro when i got a drip in it and had no problems, but i didnt know if poly will work the same way, since the coats dont react together like nitro does. thanks again,

- Louis

[erikbojerik]

You will have to sand the entire surface and shoot a coat over the whole thing again. Otherwise you will see the lines between coats.

[Ledzendrix1128]

thanks eric,

yeah i was definately planning on spraying more coats, but i was jsut concerned about how the new coat would react with the old one compared to the way nitro does, but i already got the answer to the question just using my head... kind of an impulsive post here... sorry bout that lol

[Southpa]

Gotta knock 'em down before they get bigger! When I'm shooting poly I'm extra careful with the first few coats. If I have to overly sand an area just to knock down drips and sags then I am risking sanding thru to wood or that beautiful dye/stain/paint job (shudder). And poly is VERY unforgiving that way, no way to correct and/or hide those mistakes.

Here is what I do if I get a run or sag. About 1/2 hour after shooting I will very gently cut away the offensive protruberance with a razor and try to bring it down level to the surrounding surface. Then just a light scuffing when cured and continue spraying until I get a big enough build up to sand things out a little more aggressively.

Just to add, gravity causes sags. From now on I only hang my guitar vertically during the first few shoots. Afterwards I lay it down flat and level and get much better results and less risk of getting those nasty sags and runs.

Edited December 1, 2005 by Southpa

[n8rofwyo]

Southpa: I read a post of yours a couple of months ago stating the same concept. I tried it and won't do it any other way now. Sags in the finish on the final coats fill my heart with an undiscribable(sp) hatred of gravity....especially with poly's nuance for "letting it all go".

Ledzendrix: The real bitch of poly is that as soon as you sand through it, the top coat- or finish - is blown. Nothing to do then but cuss and throw tools. It still bugs me to shoot over a mistake, but in this case its the best option.

By the way happy B-Day!

Nate Robinson

[erikbojerik]

Just to add, gravity causes sags...I lay it down flat and level and get much better results and less risk of getting those nasty sags and runs.

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Exactly the way I do it (except I never ever hang it vertically until all the coats are on, stiff, and in the cure-for-a-month stage). It helps to actually put a level on it before you shoot, so that way you know that your support blocks won't cause the clear to run out toward one corner of the body.

This way, if you do get a run, it is only a small one on the edge of the body and not as much of a pain to level out.

[Ledzendrix1128]

Ledzendrix: The real bitch of poly is that as soon as you sand through it, the top coat- or finish - is blown.  Nothing to do then but cuss and throw tools.  It still bugs me to shoot over a mistake, but in this case its the best option.

By the way happy B-Day!

Nate Robinson

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your reffering to if you sand through to the wood/paint right? as long as you only sand the poly, the top coat isnt effected correct. Cuz i got the sag after the 5th double coat i sprayed (go over the body twice at a time). I plan on waiting 72 hours (like the directions say). Im running this by you guys cuz it ALSO says to wait 3 days before buffing... and from what i hear.. .thats just a tad too quick. Someone else told me i should be scuff sanding between coats. If i do this, how long should i wait before i scuff sand? the directions say 2 days... is it really necessary to wait that long?

And thanks nate, the big 18 now.

[Southpa]

Someone else told me i should be scuff sanding between coats. If i do this, how long should i wait before i scuff sand? the directions say 2 days... is it really necessary to wait that long?

It depends on the conditions, try denting with your fingernail. If it yields a bit too easily then wait longer. The scuffing is just to make a rougher surface so the next layer bonds well to the previous layer. When the poly is still wet you have no trouble getting the next layer to stick, but when cured the surface is too smooth. With nitro lacquer you don't have to worry about scuffing because surface layers chemically fuse to those layers previously laid down.

Edited December 1, 2005 by Southpa

[Ledzendrix1128]

ahhhh i see, so you guys think 5 coats is enough to level off those sags without sanding through too easily?... or should i just spray over them till i get a nice thick build, then level it all and spray a top coat with the last can or soemthin?... ive only gone through like 1 and 1/8 of a can... and i started with 4 total... so i got almost 3 more cans to play around with. would i be better off continuing spraying clear and sanding the sags out at the end?

[Southpa]

I would try to get the sags down to near level. The more poly you add the higher those spots will be. You can't keep shooting and hope to build up "around" those areas. Whatever you add will also add to the sag height. Its a pain in the butt but one of the necessary evils . If they are really high try cutting them down with a sharp razor while trying to avoid messing up the surrounding surface. Then take some 100-120 grit and detail sand ONLY the high spots. Once you feel they are low enough, ie. sanding further will jeopardize the surrounding finish, give a general light scuffing w/ 320 on flats and 600 on corners and edges (VERY carefully) and resume shooting. Just make sure it doesn't happen again!

Edited December 2, 2005 by Southpa

[Ledzendrix1128]

yeah i dont think the sags are even bad enough for the 100 grit thing.... they leveled out since the finish cured a little bit. But another problem that i cant avoid without level sanding any is the fact that its a swirl.. the swirls themselves are adding random areas of unevenness... i dont know if this is normal or not, but im gonna just level it off a few times throughout this process of clearcoating, and it should turn out nicely.

Edited December 2, 2005 by Ledzendrix1128

[erikbojerik]

Yes with poly, if it cures at all between coats (e.g. more than tacky), you need to scuff sand between coats. Just make sure you scuff EVERYWHERE before you continue shooting...don't leave a single tiny little shiny spot, or it will lift and peel.

[Ledzendrix1128]

ok cool, its cold out here again, i should be able to level sand today around 5 (the 73 hour mark) and when it gets warm out again i will try to get it finished and post some pics

[Ledzendrix1128]

Just make sure you scuff EVERYWHERE before you continue shooting...don't leave a single tiny little shiny spot, or it will lift and peel.

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So i scuff sanded everywhere, but just to clarify.. should i also sand the edges. I know ive been told to never sand edges bc you can easily sand through. and i also didnt sand very deep, and the dust got everywhere, but when i wiped it down with water, theres still spots that are shiny. Im afraid im gonna sand though bc of the paint swirl being raised, so if i sand completely level it will most likely cut through to the paint. is it really necessary to eliminate ALL shiny spots?

[n8rofwyo]

You want to hit as many of them as you can. When I first started using it on guitars I didn't sand between coats. After getting a ding I noticed what looked like a little piece of saran wrap at the edge of the ding. I grabbed it, pulled it, and a strip of poly came off in my hand. It was just like peeling an old sticker. I had to pulls "strips" of finish off for a solid 4 hrs. just to start all over with the finish.

Try gently hitting the edges with 400 grit, it seems to work well for me as long as use a light touch.

Hope all goes well.

Nate Robinson