Across:
1 Pal out of Spinal Tap (‘pal’ out of ‘Spinal Tap’ = spinat = PAINTS) makes it more colourful on the canvas (PAINTS),
4 Rejection (BRUSH-OFF) in Brief Encounter (BRUSH -) is really rotten! (-OFF) (BRUSH-OFF),
9 Finishes 11 down similarly (‘American flag’ = ENSIGN) working on crafty type (Navy rank = ENSIGN),
10 Those who put on their coats to work (PAINTERS) with most of 1 across (most of ‘paints’ = PAINT-) in a number of hospitals (-ERs = ERS) (PAINTERS),
12 Every so often (SOMETIMES), Somme site (Somme site = SOMETIMES) is redeveloped (= anagram indicator),
13 Delete (ERASE) some disclaimer as expected (some ‘disclaimer as expected’ = ERASE),
14 1 across (paints = PAINT-S) covers 4 across, for starters (‘brush-off’ = -BRUSH-), and final course (final ‘course’ = -E) (PAINTBRUSHES) taken in hand by one working on the last supper? (PAINTBRUSHES used on ‘The Last Supper” painting),
18 One of The Cars (STOCK car) sells (MARKETS) (STOCK MARKETS) places where The Animals sold out perhaps (STOCK MARKETS),
21 First piece (preface/preamble = PROEM) of writing (from the 3Rs = -R-) in what 6 down produced (poem produced by ‘sonneteer’ = P-OEM) (PROEM),
22 Avoids shop for Cubans (NON-SMOKER avoids shop for Cuban cigars) and the one that doesn’t cure the meat (NON-SMOKER),
24 Casual (RANDOM-) labourer opening (‘labourer’ opening = -L-) yard (-Y) (RANDOMLY) without rhyme or reason (RANDOMLY),
25 American (Yank) loses the head (y) (‘yank’ without ‘y’ = ANK-) with a (-A-) trim (y) swimmer (‘ray’ without ‘y’ = -RA) (ANKARA) from European capital (ANKARA),
26 Compensate for (OUTWEIGH) women (-W-) from Aer Lingus (-EI-) in tough (tough = OUT-GH) flight (= anagram indicator) (OUTWEIGH),
27 Brie coming out of Gobi Desert (‘Brie’ coming out of ‘Gobi Desert’ = godset = STODGE) is lumpy in an indigestible way (STODGE).
Down:
1 Exercises (PRESS-UPS) journalists’ (PRESS -) union (-U-) and pressure (-P-) on society (-S) (PRESS-UPS),
2 What definitely affects the retirement plan (INSOMNIA) will keep you up all night (INSOMNIA),
3 Local drunk (TIGHT) has a habit of going missing when it’s his round presumably (TIGHT),
5 Retweet (retweet = R-T) over English (-E-) judge (-ASSESS-) contacting workers (-MEN) (REASSESSMENT) leads to another judgement (REASSESSMENT),
6 Enters one (enters one = SONNETEER) building responsible for 20 down (SONNETEER responsible for ‘sonnet’),
7 Approaches missing chap (‘approaches’ missing ‘chap’ = aproes = OPERAS) for the divas (OPERAS),
8 Red-tailed black sharks (FISHES) sound (= homophone indicator) almost vicious! (“vicious” sounds almost like “fishes” = FISHES),
11 A Crimean (A Crimean = AMERICAN) novel (= anagram indicator) linked to Tyre (“tyre” = “tire” = FLAG) reporting (= homophone indicator) (AMERICAN FLAG) old glory (Old Glory = AMERICAN FLAG),
15 Part-time office worker (-TEM P-) in plane, for example (tool = TO- -OL-), going to Spain (-E) (TOTEM POLE) for post popular with Indians (TOTEM POLE),
16 A job for the sailor (DECKHAND), held by the dealer (DECK-) and held by the gambler (-HAND) (DECKHAND),
17 Takes drugs (USER-) from celebrity (-NAME) (USERNAME) with online identity (USERNAME),
19 Old (O-) drunk in Portugal (-PORT-) finds love (-O) (OPORTO) in Iberian city (OPORTO),
20 Sent (sent = S-NET) off (= anagram indicator) over starting outburst (starting ‘outburst’ = -O-) and named (-N-) (SONNET) by the linesman (SONNET),
23 One of those in 10 across (“painters” = one painter = MONET) always starts the count (always starts the count = ‘one’, two, three = -ONE-) in Montana (M-T) (MONET).