Sonnet LXXXII

I grant thou wert not married to my Muse,
And therefore mayst without attaint o'erlook
The dedicated words which writers use
Of their fair subject, blessing every book.
Thou art as fair in knowledge as in hue,
Finding thy worth a limit past my praise;
And therefore art enforced to seek anew
Some fresher stamp of the time-bettering days.
And do so, love; yet when they have devised,
What strained touches rhetoric can lend,
Thou truly fair, wert truly sympathized
In true plain words, by thy true-telling friend;
   And their gross painting might be better used
   Where cheeks need blood; in thee it is abused.

 The objections raised against the rival poet(s) continue. The youth is being misled by them, allowing himself to be flattered with the artificiality of rhetoric. Although he is commiting no crime (yet he did vow eternal exchange of hearts), perhaps he turns aside because he exceeds in beauty anything that may be captured by the pen of this writer, and his true worth requires that he seek abroad for other talents to depict him. Yet the poet fears that this is dangerous and corrupting, for the youth does not require flattery, or artificiality. He is himself so fair that he best suits the simple truth. Let him therefore leave these false and empty poets. They had better turn their attention instead to pale and sallow cheeks which need the hypocrisy of untruth to bolster their image.

The 1609 Quarto Version

IGrant thou wert not married to my Muſe,
And therefore maieſt without attaint ore-looke
The dedicated words which writers vſe
Of their faire ſubiect,bleſſing euery booke.
Thou art as faire in knowledge as in hew,
Finding thy worth a limmit paſt my praiſe,
And therefore art inforc'd to ſeeke anew,
Some freſher ſtampe of the time bettering dayes.
And do ſo loue,yet when they haue deuiſde,
What ſtrained touches Rhethorick can lend,
Thou truly faire,wert truly ſimpathizde,
In true plaine words ,by thy true telling friend.
   And their groſſe painting might be better vſ'd,
   Where cheekes need blood,in thee it is abuſ'd.

Commentary

1. I grant thou wert not married to my Muse,
thou wert not married ... and therefore - the implication is that no breach of marriage vows is involved. The youth may look on the seductive words of other love poems and be tempted by them, without loss of faith. my Muse = me as a poet; my poems.
2. And therefore mayst without attaint o'erlook
without attaint = without being convicted of sin or crime. The term is a legal one derived from the verb 'to attain' (see OED attain.v.3.). o'erlook = look over, view, look upon. In connection with the marriage obligations which are said not to apply here, there is a suggestion of a sexual peccadillo, that of eyeing a woman up and down to assess the enjoyment of having her as a sexual partner.
3. The dedicated words which writers use
The dedicated words = words dedicated to portraying (your) beauty; words used in a dedicatory preface, as for example that of Shakespeare to the Earl of Southampton in Venus and Adonis and The Rape of Lucrece. If the latter meaning is intended, then fair subject of the next line could encompass any subject or theme of which the author wrote, and not refer to the youth himself.
4. Of their fair subject, blessing every book.
of their fair subject - 'to their fair subject' seems to make better sense. The lines however are not easy to interpret. (See the note above). If the dedicated words are simply those which describe and praise the youth, in any book, then any and every book which contains such material is blessed by possessing it. But it is not self-evident who or what is doing the blessing. It could be the youth himself, the over-arching sense being 'I grant that you may look at the words of other authors describing fair subjects, and that you may without penalty bless all such productions'. A patron, (usually one of the nobility) when approached by an author, might 'bless' the enterprise of the publication by letting it be known that he approved, and even by giving the author some money, or advancing other favours.
5. Thou art as fair in knowledge as in hue,
The meaning seems to be - 'Not only do you excel in beauty, but also you excel in knowledge and understanding, therefore you discover that etc.'
hue = appearance, colour (see 20).
6. Finding thy worth a limit past my praise;
Finding thy worth a limit past = and discovering that your worth exceeds the scope of; limit = region, territory, as in
From limits far remote, where thou dost stay. Sonn.44.
7. And therefore art enforced to seek anew
And therefore you are compelled to seek again a replacement (for the outworn poet).
8. Some fresher stamp of the time-bettering days.
Some fresher stamp = a newer, livelier form of poetry, or poet.
stamp = impression, tool for making an impression (e.g. a seal); hence, figuratively, a record, a written description, a writer.
time-bettering days = days which improve as time goes by; days which are more advanced and better than previous times. But with an added flavour of superciliousness, since we know that the poet has a low opinion of time, and the phrase is reminiscent of time-servers and other duplicitous and shallow types.
9. And do so, love; yet when they have devised,
they - presumably those who have been selected as fresher stamps.
10. What strained touches rhetoric can lend,

strained touches = artificial and awkward figures of speech, or neologisms, or new styles of poetry.
lend = provide, offer, give. As in
Disguise fair nature with hard-favour'd rage;
Then lend the eye a terrible aspect; H5.III.1.8-9.
and
And you, but one, can every shadow lend. Sonn.53.
It also has the modern meaning of 'give as a loan' which is perhaps partly activated here, giving to the art of rhetoric a slightly mercenary and usurious connotation.
rhetoric - Q gives Rhethorick. The art (or science) of making speeches. It was a major component of education in ancient times and still held a lofty place in the curriculum of the Renaissance. Nevertheless it tended to attract some scorn as being the art that enabled dullards to speak and clever but unscrupulous people to attain evil objectives. In the words of the Greeks it made the lesser truth seem the greater, and was especially open to abuse in the law courts.

11. Thou truly fair, wert truly sympathized
sympathised = accurately portrayed (so that the picture seems to sympathise with you as you are).
truly ... truly ... true ... true-telling - the repetition seems to hammer home the point that it is only truth that matters, suggesting, without actually stating it, that the other poets are liars and that the youth is deceiving himself.
12. In true plain words, by thy true-telling friend;
thy true-telling friend = me (for I alone am able to describe you as you are).
13. And their gross painting might be better used
gross painting = indecent, inaccurate descriptions. Exaggerated pictures.
14. Where cheeks need blood; in thee it is abused.
Where cheeks need blood - i.e. they would be better employed using their talents to flatter lifeless and bloodless folk who need puffing up with falsehoods.
in thee it is abused = applied to you it is misuse, misrepresentation and deception (and an abuse of your true qualities).