Bloon Liquefier is the fourth upgrade of Path 1 for the Glue Gunner in BTD6. It increases the corrosion rate of the glue to 1 damage every 0.1s, as well as increases pierce of the glue by +1. As with all Glue Gunners with Corrosive Glue or higher, the Bloon Liquefier can target MOAB-class Bloons and deal full damage on them, although the glue itself will last half as long, similarly to most types of glue.
This upgrade deals decent damage to normal MOABs and Ceramics, as Glue Gunners with corrosive upgrades can now affect MOAB-class bloons. In general, Bloon Liquefier is in awkward position in the game because it tends to lack sufficient pierce compared to other competitors such as The Big One while only being slightly cheaper.
By itself, 4-2-0 Bloon Liquefier is usually better, as it has a lot more pierce. In a few select situations, 4-0-2 Liquefier can be used to damage blimps for longer periods, although Bloon Liquefier can easily destroy non-freeplay Ceramics anyway, which renders the extra glue lifespan and extra slowdown less useful for Bloon Liquefier alone.
The 4-0-x Bloon Liquefier essentially acts as an damage-oriented Bigger Globs Glue Gunner, so it's worth taking note that crosspathing variety is improved with this Tier 4 upgrade. Bigger Globs with Bloon Liquefier is basically +50% pierce, if not taking into account other pierce buffs.
Bloon Liquefier is capable of destroying a Round 40 MOAB on its own, but this does not necessarily mean it is reliable against MOAB-class bloons. Bear in mind that glue lasts half as long on MOAB-class bloons, so 4-x-0 Bloon Liquefier is only going to do a maximum of 110 damage to the MOAB until the glue wears off again after 11 seconds.
Bloon Liquefier is much stronger than it was in BTD5 because of now hitting MOAB-class bloons, but remained outclassed by alternative effective combos. It has received some buffs in BTD6 history to make it more usable in a mid-game role and to compete with more popular mid-game options. It also received a pierce buff to make Path 3 crosspathing more practical in a real game scenario.
Initial release (compared to BTD5)
Bloon Liquefier can now target MOAB-class Bloons, although they won't be slowed and glue lasts for half duration compared to normal bloons
Bloon Liquefier costs less ($12,500 --> $7000)
Many people seem to agree 5xx Glue is 'good', but simply outclassed (mainly by 5xx mortar at a similar price). This won't completely fix that, we hope to work on making the earlier stepping stone tiers a little easier to use and provide the 520 crosspath with a better niche by making Bloon Solver benefit from all pierce twice as much (which x2x provides).
~ Ninja Kiwi, referring to buffing Bloon Solver and its earlier stepping stone upgrades
Bloon Liquefier costs less ($7000 --> $6500)
The Bloon Solver recently received a small functional rework buff, this follow-up buff will make the stepping stones to it a little easier on saving.
~ Ninja Kiwi
Bloon Liquefier costs less ($6500 --> $5500)
Adjustments to top-path upgrades to make each step more noticeable. [...] We are still looking at future improvements here, including reworking the priority tree of glue stacks.
~ Ninja Kiwi
Bloon Liquefier pierce increased (1 --> 2)
More tweaks with the intent of viable stepping stones toward The Bloon Solver. [...]
~ Ninja Kiwi
Bloon Liquefier costs less ($5500 --> $5000)
4-1-0 Bloon Liquefier (correctly) has more pierce.
Description changed from "Liquefies Bloons by popping them 10x every second." to "Can glue one additional Bloon per shot and applies liquifying compounds that do 10 pops every second."
When crosspathed with 4-0-1, there is a glob of yellow glue, even though it shoots green glue.
There was a bug in Version 23.0 where the 4-2-0 Bloon Liquefier with the "Splatty Glue" MK only has 8 pierce instead of the expected 9, as that MK grants +2 pierce for Glue Splatters. This was fixed in Version 24.0.