What exactly you are trying to do? Its not done this way. Is it wood?
I have tried all possibilities and here is my advice if wood, as possibly all other scenarios lead to a fault or mediocre quality:
1.V carve
2.Sand a bit the edges and clean the V channel perfectly
3.Find the best quality and fastest dry impregnation lacquer for outside wood. Best works water based urethane.
4.Paint more or less, paint back if needed, paint sides if needed. Dont care for inside of channel
5 Let dry
recommended time
6. Use only 2 component Urethane paint. MIx and paint the channels without caring if you make all thing dirty.
7.Let it dry for a couple of hours
8. Sand directly with 100 and up sander. If you have a good sander thats it. If not sand up to 240.
9. Arm yourself
with a sponge and apply finish with careful brushing movements so as the paint or lacquer not to go in the channel
For metal:
use only 2 component base and paint. Sand face down on flat plate, do not use polisher. Or if using polisher, do that after the base , then spray color and then fast clea with acetone very carefully the polished part that need to be seen.
Here is an illustration of the process. In fact exactly what i am doing today:
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