finish the cr-co as you wish, sandblast it with al2o3 with any size any pressure any direction(keep the nose of the sandblaster at least 5 cm away from the framework),wash it with tap water, dry it with the lower corner of your lab coat or use a tissue(if you don't feel comfortable with this, use distilled water or a steam cleaner),apply the opaque in a proper manner.
take extra care of following:
1- the air bubbles underneath the sprue( always properly solder it).
2- oil with compressed air.
3- the use of used metal.
4- thin metal areas( less than 0.4 mm is not good).
5- malfunction vacuum system of the porcelain furnace.
6- over-firing any layer.
7- sharp corners, whether its positive sharp or negative sharp( a groove like).
there are so many other mistreats of the metal but mostly they are well known.
to check the opaque bonding make a trial crown or bridge finish it glaze it, then hammer it on the occlusal side on a piece of soft wood, now look to the framework if its still have a thin layer of opaque or ceramic, then your metal treatment is good, if the metal or the oxide exposed totally after hammering thin you most likely have a problem in treatment of the metal.
to check for air bubbles occurring during firing because of the metal oxide, fire a trial crown with porcelain for ten full cycles, each cycle reduce the temp 3-4 degrees with no hold at final temp, don't remove the glazed layer as removing it will allow the gas/air to escape easy with no problem, now if you have air bubbles after those ten cycles open the air bubble and look if the metal or the oxide is totally exposed, if its exposed then you have a problem with the metal oxidation or opaque firing(many times the compressor oil will cause such a problems).
to check the greenish problem , fire the opaque, then warp the cold opaqued object with a white paper tissue, dip it in water, remove it, leave it for ten minutes, open the tissue, if you find any yellow or green spots on the tissue it is most likely because of a problem with vacuum of the furnace.
if you take care of all those issues you can grant your porcelain to metal bonding for lifetime in 99.99% of the cases.
best wishes