This question showed up in my Writing part of F.C.E. test when I wrote
TV is the most bought electric household appliance
In that moment, I wondered whether boughtest is the correct form instead of most bought.
"-Er" and "-est" suffixes are used only for adjectives, not for past or present participle forms of a verb even though they can function as an adjective. For example:
This is a sought after item. ( O )
This is a more sought after item than X. ( O ) This is a sougter after item than X. ( X )
This is the most sought after item. ( O ) This is the soughtest after item. ( X )
Boring (or Bored) is derived from the verb to bore.
It is more boring than X. ( O ) It is boringer than X. ( X ).
If you contrast it with the adjective "strong", the difference becomes clearer.
It is stronger than X. ( O ) It is more strong than X (possible but not idiomatic)