If you specify a wish that has 2 contradicting terms, “rustproof”/”fixed” etc alongside a specific object type that doesn’t suffer from erosion, there should be a chance that the wish parser resolves the contradiction by respecting the erodeproof part and not the object type part, so you end up getting an erodable but erodeproof object that wasn’t the type you asked for.

A death ray zapped over a monster egg on the floor kills it, making it so that it will never hatch.

Possibly, this effect could also be achieved by erodeproofing or canceling the egg.

Dipping an item into any fountain may erodeproof it with low probability. (Or else have this only apply to magic fountains with somewhat higher probability.)

Add as rare possible fountain effects erodeproofing and blessing a dipped object. (Possibly only in magic fountains.)

Spell of repair: level 3 matter spell. If wielded weapon is eroded, fixes its erosion. Otherwise, target a random piece of worn armor and fix any erosion it has. At Unskilled, the erosion will only be repaired 1 level at a time; at Basic, 2 levels; at Skilled, 3 levels; and at Expert (maybe) the item will be erodeproofed.

Lances go through one or two levels of visible damage (e.g. “cracked lance”) before being destroyed completely, rather than being able to break outright. Repairing the lance with the same methods being used to repair erosion will fix this damage, and erosionproofing it will make it immune to cracking or breaking.

Ring of item protection (ported from Rogue’s ring of maintain armor): protects all inventory items from damage of any sort, which includes outright destruction such as monsters casting destroy armor and ring/wand explosions. Also includes protection from disenchanter disenchantment.

Scroll of repair: uncursed repairs all levels of erosion on an item of the player’s choice; a blessed scroll repairs all erosion in the player’s inventory; cursed scroll attempts to erode one random item in inventory 1d3 times. If confused and non-cursed, will erosionproof an item of the player’s choice; if confused and cursed, will remove erosionproofing from a random item and then erode it 1d3 times.

The scroll of enchant weapon retains its confused effect of erosionproofing a weapon, but it no longer erosionproofs non-weapons or repairs the weapon. Enchant weapon scrolls are also made much less common, with the probability dumped into the scroll of repair instead.

Monster or damage type that un-erosionproofs gear, not very common.

Assuming a scroll of consecration exists: confused scroll of consecration consecrates your weapon, which blesses and fooproofs it and maxes/increases its enchantment with no possibility of evaporating. Possibly, turn it into an artifact as well.

TDTTOE: If it’s possible to fooproof arbitrary items, fireproof candles are unlightable.

Erosionproof items should be immune to disintegration as well.

Erosionproofing projectiles greatly reduces their chance of breaking.

Weapons can rarely randomly generate as erodeproof.