Casey Bever Posted February 9 Report Share Posted February 9 I am running into an issue during wet sanding, I am using a wipe on poly for the first time and I have never seen such uneven finishing at this stage. Sanding at 1000 grit dry looks great, no shiny spots. But after wet sanding to 1500 I see this? What am I missing here? Any help or suggestions would be greatly appreciated. Quote Link to comment Share on other sites More sharing options...
henrim Posted February 9 Report Share Posted February 9 I suppose it is what it is. Uneven. Maybe too thick layers? I have never coated a guitar with wipe-on. I once tried the procedure though, on a batch of test pieces, and found out it’s not my kind of process. No gain over spraying. I was hoping to free my self from spray gun cleaning. But no cigar. 1 Quote Link to comment Share on other sites More sharing options...
Bizman62 Posted February 9 Report Share Posted February 9 Hi and welcome! Yep, looks uneven to me. Did you use a block or your fingers when sanding? I've used poly only twice, the first time there was some humps and bumps but the second one came out perfectly level after sanding the orange peel off. Even the sides are almost flawless. But the guitars were flat so using a semi hard block was a no brainer. Don't know what kind of a sponge as a block would follow the curves but still only knock off the high spots. Quote Link to comment Share on other sites More sharing options...
Casey Bever Posted February 9 Author Report Share Posted February 9 Yeah, thats the weird part. After the 800 grit paper it looks great, No shiney spots, nice and flat. I rub it with a damp cloth to make sure its not just dust build up making it look flatter than it is. Then I go down in grit, 1000, 1200, 1500 and then this. Looks horribly uneven. I guess the only answer is to sand until looks good with the 1500... I just never get this when I spray laquer. So I wasn't sure if it was a wipe on poly thing, an that the polish would flow it out (optimism lol) But with guitars its what you see is what you get. Finish looks bad = bad finish. Quote Link to comment Share on other sites More sharing options...
Bizman62 Posted February 9 Report Share Posted February 9 9 minutes ago, Casey Bever said: Yeah, thats the weird part. After the 800 grit paper it looks great, No shiney spots, nice and flat I guess the main difference between wipe-on and sprayed is in the surface before sanding. I mean, when spraying you often get some "orange peel" effect, tiny bubbles bursting through a semi solid layer, leaving tiny gaps all over. Sanding them off is similar to carving a hollow top with guiding drill holes. But with wipe-on you'll get brush strokes. Long ridges at the best but if you're an inexperienced painter like me you'll try to spot fix and potentially wipe that spot thinner instead of getting it level. If memory serves me right there was some advice about thinning the poly down much more than they recommend and wipe on a million layers or even a couple less. The problem with multiple thin layers is that unless the poly melts to itself like nitrocellulose lacquer you'll see where you've sanded through one or more layers. Would a final coat fix that, I can't remember. Anyhow, brushing any lacquer or paint is dark art to me. 1 Quote Link to comment Share on other sites More sharing options...
henrim Posted February 9 Report Share Posted February 9 18 minutes ago, Bizman62 said: you often get some "orange peel" effect, I may be stating the obvious but you make it sound like it is a feature, when it is an avoidable flaw 1 Quote Link to comment Share on other sites More sharing options...
Bizman62 Posted February 9 Report Share Posted February 9 53 minutes ago, henrim said: you make it sound like it is a feature With a €15 gun and no training it's a feature. 1 Quote Link to comment Share on other sites More sharing options...
henrim Posted February 9 Report Share Posted February 9 7 minutes ago, Bizman62 said: With a €15 gun and no training it's a feature. Fair enough, and the good thing is it’s not a big deal to fix on a guitar sized object. 1 Quote Link to comment Share on other sites More sharing options...
Andyjr1515 Posted February 12 Report Share Posted February 12 Is the actual surface lumpy...or is it that you have a pretty flat surface but that you have areas that are matt finish, other areas that are glossy and contour lines between them? I'll post at more length when I get a moment to explain why and how, but with standard polyurethane finishes, you cannot sand or buff down to a gloss finish as you would with, say, nitro. I think this issue is covered in my Topic on wipe-on poly finishes in the 'Tutorial' section but no-matter - I'll explain fully in any case when I get a moment Oh - and don't worry...you won't have done ANY harm doing what you have already done... 1 Quote Link to comment Share on other sites More sharing options...
Andyjr1515 Posted February 12 Report Share Posted February 12 Just had a quick look - yes, I did cover it. See 'Stage 3' in this short Tutorial: Quote Link to comment Share on other sites More sharing options...
Andyjr1515 Posted February 13 Report Share Posted February 13 So, in summary: - With finishes such as nitro, each coating layer that is applied 'melts' into the previous coats. As such, you end up with a single, thick coating layer which can be sanded and buffed to a shine. - The types of polyurethane products available to the DIY-er don't work the same. Here, each coat remains as an individual whisper-thin layer on top of the previous ones. - Once you have built up enough layers of thickness to start flattening, then you will be sanding through multiple layers as you remove the dints, dips and dust buggies. Where you cut through to a previously flattened layer, that will show as matt. Where you cut through into a layer that wasn't previously flattened, it will show as gloss. - exactly the same will happen if you use a buffing wheel. Here, the heat will tend to melt the varnish layers but you will still get the same contour line effect So, with DIY polyurethane varnishes, I have found (but others may have other ways of doing it) that the trick is to: 1. Understand that the finished gloss is the final two or three coats you apply, unflattened and unbuffed (initially - see step 8 for more info) 2. Flatten the 'final' multi-layered coating, exactly as you have done, until it is perfectly smooth, dint and dust-bug free. I do this final stage with c 3000 grit emery, or something like 8000 micro-web, used wet. 3. Wipe clean of all sanding residue and let fully dry 4. Apply your 'final coat' of gloss 5. Let it dry fully. Any flattening matt marks showing through? If yes, then apply another gloss coat over the first one. It might need a third. 6. If there are other aberrations you are not happy with, no worry - just treat these two or three as extra pre-gloss coats. Simply repeat steps 2-5 7. Once you are happy with the dried gloss finish, STOP. Then leave it for at least 2 weeks to fully harden. 8. Once it's fully hardened you can use a good quality auto polish (I use Meguiar's Ultimate Compound) to hand buff it up using a clean non-lint cloth. You can repeat that hand buffing from time-to-time throughout the instrument's life - it may be a very thin final gloss coat, but it IS tough. Don't use an auto powered buffer - this will generate too much heat and will soften that top gloss layer. If this happens, then you would need to to start again at step 2! These are a couple of examples done this way: 2 Quote Link to comment Share on other sites More sharing options...
ADFinlayson Posted February 13 Report Share Posted February 13 Andy has the issue exactly right, it's called ghosting or witness lines which occurs with poly. Once you are happy that the paint is flat, do a last thin coat, maybe 2 if needed, then buff that. Typically with cellulose (nitro) you would spray many thin coats, 12-15 is what I normally do, but with poly you tend to spray just a few much thicker coats so when you come to level sand you never go through the coat you've just sprayed. The trouble with wipe on poly is that it's very hard to put on a thick coat with a rag or brush and have it go on level enough to not require a load of level sanding, so it gets applied in lots of thin coats like you would with cellulose but without the benefit of having the coats melt into one homogeneous layer. FYI you can thin and brush on cellulose and have the benefit of the coats melting into each other. 2 Quote Link to comment Share on other sites More sharing options...
Andyjr1515 Posted February 14 Report Share Posted February 14 20 hours ago, ADFinlayson said: FYI you can thin and brush on cellulose and have the benefit of the coats melting into each other. Great tip (and a reminder for use of a correctly-spec'd respirator and very good ventilation ) Quote Link to comment Share on other sites More sharing options...
henrim Posted February 14 Report Share Posted February 14 20 hours ago, ADFinlayson said: FYI you can thin and brush on cellulose and have the benefit of the coats melting into each other. Have you done this for a whole guitar body? For small repairs it obviously works but I would imagine it's not the easiest job to brush a large surface evenly with fast drying stuff like nitrocellulose lacquer. Quote Link to comment Share on other sites More sharing options...
ADFinlayson Posted February 14 Report Share Posted February 14 1 hour ago, henrim said: Have you done this for a whole guitar body? For small repairs it obviously works but I would imagine it's not the easiest job to brush a large surface evenly with fast drying stuff like nitrocellulose lacquer. Yes I have. cellulose flashes off very quickly when it's been atomised in a spray gun, but when it's still in liquid form it takes considerable longer, yes you don't want to hang around and get it on and not keep going over, it's no different to applying poly with a brush like @Andyjr1515 does. 1 Quote Link to comment Share on other sites More sharing options...
henrim Posted February 14 Report Share Posted February 14 3 minutes ago, ADFinlayson said: Yes I have. cellulose flashes off very quickly when it's been atomised in a spray gun, but when it's still in liquid form it takes considerable longer, yes you don't want to hang around and get it on and not keep going over, it's no different to applying poly with a brush like @Andyjr1515 does. Ok, nice. I was just curious if it really was an option. I'll take your word on it, I have no intention to try unless I really have to. Quote Link to comment Share on other sites More sharing options...
ADFinlayson Posted February 14 Report Share Posted February 14 1 minute ago, henrim said: Ok, nice. I was just curious if it really was an option. I'll take your word on it, I have no intention to try unless I really have to. No of course, the real solution is to get a spray gun. Quote Link to comment Share on other sites More sharing options...
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