Yeah, the chapter on solutions is not very clear. So here goes. You make a part "A" which is Ferric Oxalate and oxalic acid. Then you make a part "B" which is ferric oxalate, oxalic acid AND a restrainer, usually potassium chlorate. Finally you make your metal salt solutions. All of this of course according to the quantities Arentz has.
Now then, once you have all 3 or 4 solutions (depending on whether you are making pt/pd or pd only) what you do to coat a piece of paper is you mix drop amounts of each solution.
For example, if you want to coat a piece of 8x10 paper and are using the hake brushes, you want to put 24 drops of solution A or a combination of solution A and B to make a total of 24 drops.
Then you add drops of pt and pd to make a total of 24 drops. So all together the drops of ferric oxalate + the drops of metal salts would total 48 drops.
As you read further along, you will come across a recomendation Arentz makes about adding the restrainer to the DEVELOPER instead of the coating emulsion. This approach is much, much better.
Adding the evil #2 solution (what you call part B) usually results in mottled and grainy prints.
It is best to mix solutions yourself, for palladium if you buy 10 gr you can make 110 ml, this will last you for quite a while and is much cheaper than buying it mixed.
The metal salts solutions will last indefinitly, the ferric oxalate solutions anywhere from 3 to 6 months, I have used farily old ferric oxalate with no significant changes, but of course, fresh is always best. I would recommend you only mix about 15 ml of Ferric oxalate at a time, this way you will always have a fresher solution.
I urge you to get the Richeson 9010 brush. This brush will make a significant difference in your savings. If we take the example above, for an 8x10 I use a total of 16 drops! 8 ferri, 8 pd. This is more than half savings in solution than what you would get if you coat with hake brushes. Plus you only need one, not an army of hake brushes. The brush is expensive, but in the long road it will make a big difference.