Sanding Sag from BM Super Spec P22

[Patrick_]

Hi guys,

First post - I am a painting rookie here. I have learned quite a bit from this forum already including gun adjustments, basics of body work, and the value of a stainless steel cafeteria tub to assist with clean-up. I am working on repainting a piece of machinery, and I am using BM Super Spec Urethane Alkyd Enamel (P22). I reduced it by 10% with mineral spirits to get it to go through the gun (Iwata LPH50 1.0mm fluid tip). I use the touch up gun because there are several hard to reach places.

I painted the last Sunday and the job turned out good, however I have one vertical spot that developed a small sag. I was reading on this forum that enamel should be dry before attempting to sand. I went out there last night, and the paint is not longer tacky and is however it is still a little soft. I was able to dent the sag with a finger nail. The TDS shows a pretty cure time of 2 weeks. My question is can I sand it now or is it still to wet?

The machinery is a temperature controlled room, and it is set 75 degrees F. Thanks for any feedback it is very much appreciated.

Patrick

[Len]

Hi guys,

First post - I am a painting rookie here. I have learned quite a bit from this forum already including gun adjustments, basics of body work, and the value of a stainless steel cafeteria tub to assist with clean-up. I am working on repainting a piece of machinery, and I am using BM Super Spec Urethane Alkyd Enamel (P22). I reduced it by 10% with mineral spirits to get it to go through the gun (Iwata LPH50 1.0mm fluid tip). I use the touch up gun because there are several hard to reach places.

I painted the last Sunday and the job turned out good, however I have one vertical spot that developed a small sag. I was reading on this forum that enamel should be dry before attempting to sand. I went out there last night, and the paint is not longer tacky and is however it is still a little soft. I was able to dent the sag with a finger nail. The TDS shows a pretty cure time of 2 weeks. My question is can I sand it now or is it still to wet?

The machinery is a temperature controlled room, and it is set 75 degrees F. Thanks for any feedback it is very much appreciated.

Patrick

If there is NO HARDENER added then you should wait the recommended amount of time before sanding. Even after waiting non-hardened enamel stays somewhat soft and should be wet sanded using 1500 grit or finer.

Are you planning or repainting that area or sanding and polishing?

[Patrick_]

If there is NO HARDENER added then you should wait the recommended amount of time before sanding. Even after waiting non-hardened enamel stays somewhat soft and should be wet sanded using 1500 grit or finer.

Are you planning or repainting that area or sanding and polishing?

Hi Len, I didn’t use any hardener. The TDS has no information about sanding, however I did call the BM tech line and they said I could sand it a couple days after spraying. I was hesitant because of what I read about enamel.

I was planning to respray after sanding because the phone tech said that it would not polish like a traditional auto body enamel. Also, they said that wet sanding would compromise the paint integrity. If I am planning to respray do you think it is still to early to sand?

[Len]

Hi Len, I didn’t use any hardener. The TDS has no information about sanding, however I did call the BM tech line and they said I could sand it a couple days after spraying. I was hesitant because of what I read about enamel.

I was planning to respray after sanding because the phone tech said that it would not polish like a traditional auto body enamel. Also, they said that wet sanding would compromise the paint integrity. If I am planning to respray do you think it is still to early to sand?

If the work has been kept warm you may be able to sand now but then wet sanding prior to spraying would not be as good as dry sanding. I'd probably dry sand with some 800 or finer then allow it to sit for a day or two before repainting.

[Patrick_]

If the work has been kept warm you may be able to sand now but then wet sanding prior to spraying would not be as good as dry sanding. I'd probably dry sand with some 800 or finer then allow it to sit for a day or two before repainting.

Got it. I will give it a go and report back how it went. One last question, because the machine is a large cast iron mass, I will have to blend the spots I am touching up. When it dries will it be obvious where the wet edge transfers to the dry edge, or should it lay down flat and not be noticeable?

Thanks again for all the help.

[Len]

Got it. I will give it a go and report back how it went. One last question, because the machine is a large cast iron mass, I will have to blend the spots I am touching up. When it dries will it be obvious where the wet edge transfers to the dry edge, or should it lay down flat and not be noticeable?

Thanks again for all the help.

Because a hardener was not used the chance of a bad reaction is possible. When blending over a hardened paint I would normally recommend applying the paint then over-reducing the paint to blend the edge of the spot then spraying a little straight reducer to melt the dry edge into the existing paint. However with fresh, non-catalyzed paint as your foundation your chances of causing the existing paint to wrinkle are much greater.

[bmarler]

len is spot on regarding the possibility of wrinkling. when i shoot equipment paint i will either, shoot it all within a couple of days, or make sure i let it sit for a couple of weeks before doing any touchup. sanding off the run will let you see how hard it is, and if you can still mark it with a thumbnail, it's not ready. especially if you're adding a bunch of thinner. most equipment will have good hard edges or breaking points where you can mask things off and protect the good paint from the overspray from doing touchup. then you don't have to try to blend it, you just re-shoot it.
i shoot equipment with a full size gun typically, with around a 1.4 nozzle. i'll go in and shoot all the nooks and crannies first. typically give them two coats, letting the first coat get pretty well dry before adding the second coat. then, when that is still soft, i'll go over all the rest of the machine, pretty much ignoring all the stuff i already shot. the trick is to get it covered while the original coats are still soft enough for the top coats to bite in and blend. doing it this way really reduces the possibility of sags or outright runs. i don't know if you've heard of them, but the dekups style cup liners will allow you to actually shoot with the gun upside down. this can be a game changer for painting equipment due to all the inside corners and holes and frames.
i'll usually use a fairly hot reducer too, so it flashes quickly. fast reducer (like lacquer thinner) can decrease the gloss a little too. some paints won't take it though, so do a little testing first. same thing with hardener. most equipment paints respond really well to the addition of hardener. i use cheap acrylic enamel hardener. the paint cures much faster and holds gloss better. just do a test batch first whenever you're playing junior chemist. if it's not going to work, you'll see it right away.

[Patrick_]

Thanks for the replies guys. I went out there last night and dry sanded it down with 1200 grit (that is what I had on hand). It seems that it is still soft, but not clogging the sand paper. With the 1200 grit, it came off in thin slivers/shavings. I think I was able to get the run out, but I am not sure how to clean off the dust the shavings. I didn't want to use tack cloth, and it sounds like a quick swipe with mineral spirits would be a bad idea. If I am understanding correctly, it sounds like I should avoid thinner as it can cause wrinkling when reacting with the original coat. I can get most of it off with the air gun. Also, I would normally just wait a couple months for it to complete dry/cure, but I have a temporary spray booth in my garage that I need to take down to make space for a piece of equipment that is coming in the next week or two.

Those dekups sound like just what I need. I was using the touch-up gun because I can rotate the cup upside down. Unfortunately, this piece of machinery has more sweep curves that sharp edges. I could probably still do it, but the would be pretty big sections.

The plan is for now:

Clean the surface (Hopefully you guys have a recommendation)
Wait until tonight to reshoot (24 hours after sanding) first coat
Shoot a second coat tomorrow per TDS (8 hours after first coat)

If this sounds like a bad plan, I will modify.

I really wish that I had used hardener, however I was unaware that I could use with this paint. I thought that it was strictly for the automotive paints. I didn't see anything mentioned in the TDS either so I just went for it. Lesson learned.

[bmarler]

Thanks for the replies guys. I went out there last night and dry sanded it down with 1200 grit (that is what I had on hand). It seems that it is still soft, but not clogging the sand paper. With the 1200 grit, it came off in thin slivers/shavings. I think I was able to get the run out, but I am not sure how to clean off the dust the shavings. I didn't want to use tack cloth, and it sounds like a quick swipe with mineral spirits would be a bad idea. If I am understanding correctly, it sounds like I should avoid thinner as it can cause wrinkling when reacting with the original coat. I can get most of it off with the air gun. Also, I would normally just wait a couple months for it to complete dry/cure, but I have a temporary spray booth in my garage that I need to take down to make space for a piece of equipment that is coming in the next week or two.

Those dekups sound like just what I need. I was using the touch-up gun because I can rotate the cup upside down. Unfortunately, this piece of machinery has more sweep curves that sharp edges. I could probably still do it, but the would be pretty big sections.

The plan is for now:

Clean the surface (Hopefully you guys have a recommendation)
Wait until tonight to reshoot (24 hours after sanding) first coat
Shoot a second coat tomorrow per TDS (8 hours after first coat)

If this sounds like a bad plan, I will modify.

clean and wipe with ppg dx330 wax and grease remover.
cross your fingers, if it starts lifting or wrinkling, just stop. it will only get worse.

[Patrick_]

Thanks bmarler. I will be definitely keep my fingers crossed on this one. The only wax remover that was at my local paint shop was PCL 2009 bug and tar remover (I'm in CA). You mentioned that you use lacquer thinner to reduce machinery paint, how much do you usually reduce it? Just curious. I am reducing down to 10% with mineral spirits.

[bmarler]

Thanks bmarler. I will be definitely keep my fingers crossed on this one. The only wax remover that was at my local paint shop was PCL 2009 bug and tar remover. Being in CA, it can be hard to a lot of options. You mentioned that you use lacquer thinner to reduce machinery paint, how much do you usually reduce it? Just curious. I am reducing down to 10% with mineral spirits.

i reduce to viscosity, not really a measurement. i can tell, when i move the paint stick through the mixed paint, it moves a certain way. there's a little wiggle to it. i imagine it's less than 10%, but all paints are different. i'll dump my mixed paint back into the gallon too, so it slowly becomes more reduced as i use out of it. you can only do that if you're not using hardener.
there's no way i'd use lacquer thinner on what you have now though, it's almost a guarantee you'll have wrinkling. you want the most benign reducer you can get. mineral spirits or maybe xylene. xylene is a little cleaner, but both are fairly slow.
also, there are some "modified" enamels that don't mix well with lacquer thinner, so do your testing.

[Patrick_]

Quick update:

I went outside and wiped the spot down with wax and grease remover, and I noticed that part of sag went deep than I thought. Either that, or the paint was tearing a little - seen in the top part of the transition from the paint to the filler. Shown below:

InkedScreenshot 2022-04-24 135348_LI_circled.jpg

I sanded a bit more with the 1200 grit, and got this far. It is close, but there the section isn't quite a smooth transition. I am hoping the paint will cover when I go at it later this evening. I am planning to spray over the 1200 grit scratch marks unless I should give it more tooth. I originally final sanded the filler with 400 grit.

Screenshot 2022-04-24 135441_circled.jpg

b marler,

I just saw your post, and I will stick with mineral spirits moving forward. Xylene is another one that is on the CA no-no list. Thanks for the help.

[Patrick_]

I was able to reshoot last night after sanding down with the 1200 grit, and so far it has worked out very well. The sag is no longer there, and I have not seen any wrinkling as of 20 mins ago. I kept the temp. in shop set to 77 deg F all night. I ended up reshooting the whole section rather than trying to blend the small spot. Hopefully, if the paint was to wrinkle, it would have happened already. Thanks for the help.

[bmarler]

good deal, if the paint was going to wrinkle, it would have done so by morning. i think you're out of the woods now.

[Patrick_]

It has been a couple weeks since I repainted the area, and I believe that I may have over-reduced the paint in an attempt to get the paint through the 1.0mm fluid tip. I noticed that the paint in the areas that I touched up is showing less gloss.

For reference I reduced with 20% mineral spirits (viscosity was 22s from a #4 ford cup).

I am thinking about reshooting these areas, and reducing only by 10%. (viscosity was 52s from a #4 ford cup at this reduction). Do you have a fluid nozzle recommendation? From my research, it looks like I should try a 1.8mm.

Also, given that the paint has had a couple weeks to dry, do you think I could recoat using a catalyst enamel hardener after lightly sanding or would this be a recipe for wrinkling?

Thank you for the help.