Weblio is a very useful resource aggregator, freely available online, and sourcing from various dictionaries.
Their thesaurus entry for 錆が浮く may help you discern the actual meaning.
As far as the phrasing goes, it's important to note that one of the senses of 錆 is:
(Sourced from Shogakukan's 1988 国語大辞典; emphasis mine.)
The bold part is consistent with pretty much all senses of 浮く and 浮ける: something comes to the surface. This is true for the common English translation of "to float". However, in cases like rust on a piece of metal, describing the rust as "floating" is clearly not the correct turn of phrase, and you should probably try some other wording, perhaps even something simple like "rusted".
Update: Meaning and Translation
I may have gotten the wrong end of the stick at some point (and if so, I hope that a native-J contributor might chime in), but when it comes to the phrase 錆が浮く, I really think that translating 浮く here as "float" is missing the mark. Have a look at Google Images for the phrase "錆が浮いた" and you'll see lots of hits related to things simply "being rusted". There's nothing about "floating" at all, not even about rust flaking off. Nor is there anything about "loose". This is one good example:
Steadily polishing a rusted chain
As you can see from the image, the rust is quite minor, with no flakes, and no loosening of the surface in any way.
Update: Using Weblio
I'd mistaken your language ability. I'm sorry for that. I'd like to encourage you to work on your Japanese comprehension until you can use monolingual resources -- you'll learn a lot more about any language when learning about it "from inside", as it were. :)
In addition, Weblio has many different resources, not just a thesaurus. For instance, if you click through to the thesaurus entry, then look across the top of the page, you'll see 英和・和英辞典, which links through to the E>J / J>E dictionary page for that entry. Sadly, there is no E<>J entry currently in Weblio for this phrase, but you can use the search bar there to find other things. Looking up the E<>J entry for 浮く, for instance, can lead you to the list of usage examples, where we find that 浮く does not necessarily translate directly to just "float".
Even with limited Japanese capability, if you run across a page like the thesaurus entry, try picking apart what meaning you can directly. Also look for clickable terms -- many of the 類語, or synonyms, have clickable thesaurus entries of their own, some of them with full corresponding E<>J entries.
Good luck with your studies!
Update: Relative Clauses
In Japanese relative clauses, functionally equivalent to English phrases like the bolded part in "the thing that does XYZ", the subject of the clause can take either が or の without changing the meaning.
As such, these two are semantically equivalent:
In both cases, we have 錆[の・が]浮いた modifying (describing) the noun 鉄扉.
When converting a relative clause (that modifies a following noun) into an independent clause (a statement all on its own), we change what we're talking about -- we go from talking about the 鉄扉, to talking about the 錆, so we need to mark the 錆 as the subject using が, rather than a noun modifying something later in the sentence, when we can use の.
If you see a phrase in a relative clause that uses の after the clause's noun, and you'd like to look it up online to confirm meaning and usage, search with both の and が to catch more hits.
Other posts have gone more deeply into the mechanics of using の and が in relative clauses. This is probably the most relevant that I've found:
You might also enjoy perusing the other search hits: