Sticky residue is normally the flux. IPA doesn't dissolve flux, it just makes it runny. So if you gently wipe it off, all you are doing is smearing flux everywhere. Once the IPA evaporates, the flux turns into a horrible sticky mess.
The solution is to add a bit more IPA to the board, wait a minute or so for it to soften the flux, and then using something like kitchen roll, wipe in small circles, changing which bit of the kitchen roll you are using as you do (so that you don't smear what's already been wiped up back onto the board).
For intricate places around ICs, you can use the same process, but rather than wiping the kitchen roll on the board, use something like a pair of tweezers to push the roll down on the board and move back and forth around and between pins.
You should find the areas that have cleaned well stop being tacky. If you find anywhere that is still a bit sticky, then simply repeat the process.
What is also worth doing before you start cleaning with IPA is to gently chip off any really large blobs of flux - you can usually do this with a blunt instrument and as long as you aren't jamming something into the board it won't do any damage to the solder mask. The advantage of this is it means less flux on the board that has to be wiped off after it has softened up, so less that will smear around everywhere.
I've used OSH Park before, and never had an issue getting the boards clean with IPA and some elbow grease. As to why your particular board is being more stubborn to clean, it happens from time to time. Maybe you had ended up with more flux on the board, or when you were wiping off the IPA you used something that wasn't doing as good of a job of absorbing the grot. Could be many factors.
I've used raytech magic gel which is one of the more commonly used re-enterable potting compounds.
The mixed compound starts off with a consistency like that of a weak sugar syrup,
after a while it sets into a fairly tough greasy (with silicone) jelly. you can bounce
things off it and it's fairly resistant to being torn.
To get it off you tear or cut through the gel to get the board or wires out, and then clean the remainder off. I used Iso-propanol and compressed air which did the trick fine.
The videos on their site show what the stuff is like to work with. I'd recommend it - at the cost of silicone all over my lab and hands, it's saved me having to rebuild things from scratch.
The other reason to use it is the chemicals are much better for aquatic life than the typical bis-phenol-a based potting compounds.
Best Answer
Acetone can dissolve some plastics, so I wouldn't use this unless it was carefully tested on those specific boards.