Hi Guys,
I have been involved in the photoetching industry for almost 20 years, so I should be able to answer any questions you have. I have not seen the Micromark kit, but can offer some general suggestions.
1. Your metal must be clean before masking. This is the most important step! Even fingerprints can ruin the job. Wiping with acetone should be sufficient if you are etching brass.
2. Maintain your chemistry. If you are heating the etching solution, you will lose water to evaporation. This will affect your etch rate and undercutting. Here at the shop, we control the specific gravity ( density ) but at home you can base it on volume for the small amounts of metal you will be adding to the solution.
3. I assume the kit uses ferric chloride. Etching stainless steel will eventually kill the bath. Etching brass will turn to cupric chloride over time. Thais is not a bad thing as cupric chloride will etch brass nicely. The bath will still die over time. Adding small amounts of hydrochloric acid will extend the bath life, but you may be best off chucking the bath when it no longer cuts quickly or cleanly.
4. Ferrric chloride cuts best when heated. Just watch out for the fumes. They will etch any metal in the room. Ferric chloride is not super hazardous, but exercise care.
If anyone has any specific questions, I will be glad to help.