So, alchemical silver:
Cost ranges from +2gp for a single piece of ammunition up to +180gp for a two-handed or double-headed weapon. Nothing in the description above suggests the effect created is temporary and nothing specifies the process has to be applied during the crafting of the weapon. Based on what's written above you can take a regular steel sword to someone familiar with the alchemical bonding process and permanently "silver coat" the weapon.
Now, silversheen (as defined in Qadira, Gateway to the East):
For some reason this has a flat cost of +750 instead of being based on individual ammunition or weapon size, not sure why. The language here does indicate the weapon needs to have been crafted from silversheen originally, and the added cost makes sense: the weapon now counts as masterwork and has antirust protection.
Finally, silversheen (as a Wondrous Item):
Why would anyone pay 250gp for paste that lasts only an hour, when for less cost you can apply the aftermarket alchemical silver process to as many as 12 light weapons, 2 one-handed weapons or a single two-handed or double-headed weapon?
Why is silversheen both a special material and a wondrous item?
Why does the wondrous item work like a different special material? (It acts like alchemical silver, which has only some of the properties of the silversheen special material.)
Alchemical silver weapons do less damage so you might not want it on your primary weapon. That said, there's no reason not to carry a dagger or short sword or something similar that's silver.
The names were probably named by different people.
Alchemical silver weapons do less damage so you might not want it on your primary weapon. That said, there's no reason not to carry a dagger or short sword or something similar that's silver.
It's only 1 point less damage, and one of the characters in question uses an oversized bastard sword (2d8 base damage.) It's more of a consideration for the other character who TWFs with Small rapier and shortsword; but he's got a pair of scabbards of many blades so he can pull either the alchemical silver pair of weapons or the adamantine pairs as the situation requires. With STR-boosting magic, weapon-enhancing spells, precise strike, favored enemy bonuses, etc. the damage output can generally survive the loss of 1hp.
If I'd had the option to start with silversheen (material) weapons early on, I would have, but wasn't aware of it a material when we started. We've been cash-poor throughout the [Carrion Crown] AP, so I can't afford to lose money on the sale of magic silver weapons to purchase non-magical silversheen ones and have them re-enchanted. At the moment I'm just trying to figure out why, with two good permanent options available, anyone would spend more money on a temporary solution?
Silversheen came up during one of the Carrion Crown books and there was confusion about exactly how it worked; the GM doesn't use the online SRD and none of us knew about the special material, so I think there may have been actual silversheen weapons in the module and we were given doses of silversheen paste instead. I'm not going to look into that until we finish the AP.
Also note that mithral counts as silver for purposes of overcoming silver damage reduction, but doesn't indicate that it reduces a weapon's damage. It is a fourth option for overcoming silver DR, though a pricey one. Mithral is a Core Rulebook material, so may be available in some games where supplements aren't allowed.
Silversheen is likely both a special material and a wondrous item because someone didn't check to see if the name was used already. Just consider them two different items that share a name, as that's really the case.
The advantage of the "silversheen paste" (likely a better name for it than sharing the name "silversheen") is that it can be applied to whatever your weapon is, so if you have a +2 keen frost cold iron battleaxe, you can pute the paste on and it acts like a +2 keen frost alchemical silver battleaxe for the duration of the encounter.
The paste allows you to keep your weapon's other properties and only change the effective base material, instead of having to carry a second silver weapon and get it enhanced with a bunch of weapon properties as well.
Actually, I think I like the name "silvercoat paste" better than "silversheen paste," but you can call it whatever you like. The point is they just messed up and named two different but similar things the same name.
Qadira was also the first non-3.5 Player's Companion. It's possible that some of that material was written before final details of the CRB were in place and it was maybe an unintended oversight to have them both named the same.
The difference seems to be the immunity to rust on the silversheen material.
Lastly, the paste weighs a lot less than most weapons made of alchemical silver. A hold out dagger? Sure, those make sense. (And how many APs/modules have you seen that give out a silver dagger as low level loot?) But duplicating a main weapon will lead to inventory woes for many characters. I would only invest in an alchemical silver weapon if I was reasonably sure that damage reduction was going to be fairly common.