Every mold is going to have some imperfections - until you look at a casting you can't tell which ones matter. Sometimes dents or bumps in the mold do't show at all in the casting, sometimes they show a lot but are hard to find on the mold.
The most serious defects are those that keep the casting from coming out cleanly (or at all). You should take your test casting and hold it next to the mold to figure out what changes are needed. You may have to look closely to tell if a rough part of the casting was due to the mold or simply didn't have enough pulp in it. An Optivisor or reading glasses can be helpful.
Where a bit of pulp stuck, you may have packed it in too tightly or you may have roughness or undercutting that is grabbing the paper.
There are two main ways to fix; scraping away and adding clay.
If there are bumps or rough areas, you can gently and gradually scrape with the x-acto knife, being careful not to scratch up your smooth surfaces with the tip. If you have done some real undercutting, you may want to scrape away the top that overhangs, but more likely you will want to add in clay. Roll tiny snakes of softened clay, smash them down in with your clay shaper, smooth away the excess with the clay shaper or other ceramic tools. If you add clay you will need to bake again, but if it is tiny bits you may only need 5-120 minutes. You can keep test casting and improving, but making too many times can really add to air bubble bumps.
If there is part of the mold that you cut wrong or just don't like, it is possible to fill it in with clay and cut/impress anew - what tools depend on whether you are working the baked or unbaked parts. You can carve into baked clay but be careful, you can chip off pieces you didn't mean to. Liquid Sculpey can be used to fill in or smooth a lower part of a mold for major repair (like when you cut a part through too many layers in a multi-layered project)
If parts of the design are too tight or narrow and it was hard to get pulp in or out, you can open up shapes without changing the design much by gently scraping more of a "V" shape, widening the top but being careful not to cut into the bottom layer. You can also cut or scrape along edges to make your letters or pattern more even or more bold.
If pieces of the mold (counters in your letters usually) came off into the casting, you might have left waxed paper under them. This rarely happens if you make sure it is cleaned off and use some Sculpey diluent/softener under small pieces. Sometimes after long use things will break off. You can reattach - the best way I've found is using a tiny bit of superglue gel. You can use a bit of clay and rebake, use E6000 or jewelry glue, even a gluestick for temporary (one-time) fix.
Feel free to send me close-up photos and ask for advice in fixing problems in your mold.
It may be that you like it just fine-- after all, this is hand-made art, so as long as the casting comes out, you might not want to do anything else to it - and often practice in filling the mold is the best way to get a sharper casting.
Remember, I'd be happy to come teach you in person, we'd do several small projects to "get the hang of it" and learn various techniques, then prepare designs and do a larger, individualized project.
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