MIMF

I've been using KTM 9 & having the same problem repeatedly. After curing for a couple of weeks
I begin to color sand & in the dark color areas I can see, what looks like, I'm sanding through layers.
On my last guitar, I used Sherwood Acrylic Lacquer. I had no problems except I don't like using Lacquer.
This time I tried Target Coatings EM6000, & the same problem raises its ugly head. EM6000 is supposed
to "Burn In" which I assume to mean melt into the previous coat.
I assumed, with KTM 9, that I must not have been sanding properly between coats when left overnight.

Can anyone offer any advise?

Both of these are "legacy products" that were very popular 10-20 years ago thanks to several persuasive articles published at that time. Coatings are constantly evolving and the latest generation aren't problem free but you may find them much easier to use and much more durable. Rather than learning all the tricks to finesse these older formulations I'd switch horses and learn the tricks for the current generation. That said they will all soon be replaced by something even better that some desperate luthier will have to stumble upon, figure out, write the article and wait for others to adopt, finesse and report back, starting the loop all over again.

As David suggested, you may want to switch products.
I used Target EM-6000 with a great deal of success on at least 8 guitars.
I am now using a product called "Britetone".
It is a water base lacquer that builds more quickly than EM-6000, and doesn't have that tinge of blue.
I'm not real sure of what is going on with the product you are using, I can only say that for all of the products I have tried, there is a learning curve.
It generally takes some practice to get it right.
So to repeat the old saw; practice on scrap.

Unfortunately no modern products that I know of will burn in once they start to polymerize, i.e. crosslink. That process starts within minutes or hours after application. You will get witness lines if you sand through from one day's coating session to the next so you need to get to a level, grain-filled surface as quickly as possible and build up enough coats in your final session to allow you to level it, let it shrink down and hopefully buff it out before it becomes rock hard.

Below is a link to the product I am currently using.
I do like this lacquer as it requires a few less coats.

I will say it is pricey but in my opinion, worth it.

http://www.crystalac.info/musical-instruments.html

As David says, such 'witness lines' come about when you put a coat on top of one that has cured too much, so that there is not a good chemical bond between coats. I run into this all the time with oil-resin varnishes. A thorough light sanding, with #400 dry paper helps a lot, by raising the 'surface energy', but is not a sovereign cure. The only real preventive is to re-coat as soon as possible. That's even a good idea with nitro; the less time it has to outgas the better it burns in. With my current varnish I find that putting on two coats in a day helps a lot, even though the instructions say '24-hours'. If I could manage to do three that would probably be even better. I've gotten too used to a good night's sleep, though....

Thanks for the help guys. Nice to talk to someone who's been there.
I'm looking forward to trying something new so I appreciate your suggestions.
How does Crystalac compare to General Finishes - any opinions?

In light of what you’re all saying I did apply a coat every hour for 5 coats a day.
What will be required for the new finishes?

With every water base product I have used, they suggest only 3 coats per day. Yours may be different.
I usually wait for 60 - 90 minutes between coats.

The next day, I light sand using either 400 or 600 grit sand paper. If there are any runs, then 400.
This sanding is not to perfectly level the finish, but to give the under coat some grit.
3 more coats the second day, sand next morning, and continue with 3 per day until satisfied that the build is thick enough.
For me, it is usually about 7 -8 coats using Britetone. The EM-6000 I used to use, took about 10 -12.

The only way this schedule gets interrupted is if there is a run that I have to let dry, and sand out the next day.

Great - Thank you all. I'm looking forward to my next finishing job

I'm the last person around here who should be offering any advice, so take anything I say with a very large hunk of rock salt.

I've been experimenting with General Finishes High Performance water-based lacquer. Working on a test piece, I went through a schedule similar to what Gordon describes, letting the finish cure for about a week, and ended up during the final sanding going through the finish in a couple of spots. I began again on top of the finish that I'd sanded through, using the same three-coats-per-day and level-sanding with 400 after every three coats schedule. I let it cure for a week again, and managed to NOT sand through on the final sanding.

I expected - given this horrible treatment - to encounter witness lines galore, but did not.

Brian Evans recently observed of Enduro-Var, another GF waterborne lacquer, that it "is a non-catalyzed finish. It dries, it doesn't chemically change during a curing process the way a pre or post catalyzed finish does. This is of course what gives in the same 'burn-in' capability as nitro-cellulose lacquer". Whether this might apply to HP or not, I have no idea.

I can't find any documentation to that effect, but I do note that the specifications for High Performance on GF's website cite dry times as:

• Touch - 30+ min.
• Recoat -2+ hr
• Light Use - 7-10 days
• Cure -21 days

So, I'm guessing that the fact that this product takes about three weeks to fully cure may indeed allow for more burn-in between coats than other waterborne finishes.

I let enduro var harden for several weeks in a brush once. My go-to brush cleaner, straight acetone, wouldn't touch it. I ended up leaving it in a sealed plastic baggy for a week submerged in acetone and eventually the cured finish broke down enough to flake off the brush but the brush was never the same and I can assure you that Enduro-var as of last year was/is a finish that cures to a state that's pretty darned resistant to dissolution.

It is my understanding that the HP is a water clear finish as opposed to E-V which has an amber tint. I think there are other significant differences regarding hardness as well.

Just checked the GF site, and the curing times are almost the same for HP vs EV, but they cite a Koenig (or Konig) hardness of 59 for HP and 123 for EV. I asume that the higher number indicates a harder finish, but I'm unable to confirm that.