It has gone rather cold and damp for spraying nitro. Too cold and/or humid and you get water vapour condensing and being trapped in the finish, which isn't a good look. Spraying with a gun, you can add anti-bloom thinners, which helps, but it can only do so much.
If spraying from a can, I'd put the mask on, and spray in a pre-heated shed with the door closed. Turn the heater off before spraying. Then leave the shed, shut the door again for 5 minutes or so, during which time the nitro will become touch dry and then open the door to vent the shed for 30 mins+. You can then shut the door and warm up the shed again. It's not ideal, but as long as you keep the interior over 16°C and it feels dry, it should be OK.
I'd certainly do a test spray on a piece of wood first before spraying the guitar, just in case you do notice any 'bloom' occurring.
I'd also try and rig up an internal spray area with old sheets or cardboard to stop the overspray from spreading all over the shed.
Thanks Simon - I’ve got a humidity meter which I’ll put in the shed. What humidity level should I be aiming for?
I’ve got some extra cans so good idea to do a test spray first… looking at the forecast it looks like it might be settling down over the next few weeks?
Wednesday onwards currently seems to be reasonably warm. I'd certainly want no more than 60% RH (cheap humidity meters are fairly inaccurate (about ±10% RH) so best to be on the safe side.
Outside air %RH is forecast to be around 80%RH, as all the recent rain is keeping things damp, but warm the air up to 20°C and it drops to about 58%, which should be OK.
Today I manned up to spraying - I've been apprehensive about doing it so found lots of reasons to put it off.
Last night I hung up some plastic in the shed and moved out anything I definitely didn't want overspray on. This morning I was up bright and early rewatching the YT vids before making a start.
My routine was to heat the shed to about 20C whilst the can was heating in some warm water. Then suited up and shook up the can for 2 mins. Against Simon's advice I sprayed with the door wide open and then shut it immediately after spraying. Despite having a respirator, goggles, gloves and suit, I was super conscious to give myself as much ventilation as possible. The door stayed shut for five minutes and then open again for 30 mins to an hour.
Following this pattern I got 5 coats of sanding sealer on before the sun started to go down and I reckon another 3 coats tomorrow should do it. My plan is then to leave it until I get another calm sunny day before spraying the top coats. In the meantime I'll sand any rough areas - 1200 wet sand, sound about right?
The spraying seemed to go okay - I tried to remember all the things I'd learnt here and from the YT vids. But as with doing anything for the first time, I'm sure mistakes have been made that I won't find out about until much further down the line.
Here's my setup in the shed taken before coat 3 - The plastic has fallen down a bit in that shot but I fixed that back up:
Here's the results after 5 coats:
Not much to see - but it has a slight sheen and there's no evidence of any blushing that I can see - but perhaps that doesn't happen so much with the sealer? Fingers crossed!
Last edited by Groovyman32; 20-11-2022 at 06:17 PM.
Ive wet sanded the sealer with 1000 and it feels lovely
One thing though - on the back, in one small area it seemed to blister. I’m guessing this is a temperature or humidity related issue perhaps? Moisture under the sealer?
If that’s the case then its made me think again about spraying the gloss until the gods bless us with better conditions.
For the time being Ive restained it and I guess I can reseal it? Or can I just spray the clear over the top?
Blistering normally a reaction to something on the surface, though I have had bubbles in the finish because I’m sure it got too hot in direct sunlight in the summer. Although it’s still relatively mild here I doubt it’s due to excessive heat. So I would suspect something hot onto the wood. Humidity can cause clouding of the finish but not blistering.
I’d like to be more technical but sometimes things like this just seem to happen and without a full chemical analysis of the surface, you’ll probably never know just why.
You can spray the clear without putting on more sanding sealer, but it’s going to sink in a lot more on that patch. You’ll just need more coats on that area.
I’m wondering if it was a crack that the water got into when sanding? And that made it blister? I didn’t notice it before I started.
I think we’ve got some more settled weather on the horizon so I might just spray the gloss over a few days - perhaps one or two coats a day. That will give each each coat a good while to dry. What do you think?
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