Sandblast the case first! use the dsign glaze paste-powder over those areas, to seal the cracks, or just use all over to get an even finish. Fire at you bisbake as sixonice recommended using a long slow cool, maybe 5 minutes, not sure how long with dsign. May want to try a lower low temp, hate to see the porcelain pop off as its climbing into your furnace.
Even though I stopped using dsign several weeks ago, I used their dsign alloys for the fear of having problems.......I never had any checking-cracking with dsign 59...and still using it today.....
I also learned years ago that some alloys with matching cte for dsign ,don't work for some reason. If you are using a dsign alloy, I bet money your porcelain is unsupported. If you aren't using the dsign alloys, either change to them, or change porcelains.
Good luck, and don't touch the porcelain until its room temp after its fired!
Charles