With due respect for my fellow professionals, I hold followings views:
1. Activity of 'Painting' of various buildings so to showcase "Social Awareness Campaign (Swachh Bharat Abhiyan)" does not fall within the definition of 'Works Contract Services' which is defined u/s 2 (119) of the CGST Act, 2017 as under:
“works contract” means a contract for building, construction, fabrication, completion, erection, installation, fitting out, improvement, modification, repair, maintenance, renovation, alteration or commissioning of any immovable property wherein transfer of property in goods (whether as goods or in some other form) is involved in the execution of such contract".
1.1 Reason behind my views is that intention behind this painting activity is NOT for "building, construction, fabrication, completion, erection, installation, fitting out, improvement, modification, repair, maintenance, renovation, alteration or commissioning of any immovable property" but to display advertisement on an immovable property (i.e. on 'Housing wall, Government School and Hospital place' etc).
2. If said activity indeed falls under 'Works Contract Services' (though I feel otherwise), one needs to check possibility of claiming lower gst rate of 12% (i.e. 6+6) as per 3 (vi) of Notification No. 11/2017-Central Tax (Rate) as amended from time to time.
2.1 Considering my views explained in Para 1 above, I am not making deep drive into possibilities of lower tax-rate explained above. Only reason for putting this to let the querist know various facets to the issue raised.
3. Exact HSN code for this activity may not matter much, unless and until querist intents to claim gst-rate lower than 18%.
4. This is a Composite supply of goods and services. If the value of supply of goods constitutes not more than 25 per cent. of the value of the said composite supply provided to the Government, I would suggest through review about potential availability of exemption under Serial No. 3A of Notification No. 12/2017- Central Tax (Rate) as amended from time to time.
4.1 Without going through entire contract, concerned state's laws regarding devolution of power and responsibilities to Municipalities & Panchayat as per Article 243W (a) & 243G of our Constitution etc., it would not be possible to give any definitive views about availability of above-said exemption. Moreover, one needs to understand risks and rewards before claiming exemption and same also depends of individual's risk appetite etc.
4.2 This issue of availability of exemption (or for that matter, claiming lower tax-rate if possible) becomes much more important if concerned contract with government for subject supply is inclusive of applicable taxes (if any) and querist will get same amount in totality for its supply irrespective of either lower tax-rate or availability of exemption.
5. Lastly, if above-said exemption is indeed available, then & technically speaking, querist is not entitled for any ITC but needs to pay taxes so collected to Government. Reference/s: Explanation given to Section 11 of CGST Act, 2017 and definition of 'exempted supply' given u/s 2 (47) (Hereto, there is view available that there are no restrictions to avail ITC against goods / services used "exclusively" for exempt supply. But, same is too bold a view and same is definitely against intention of Govt. Hence, same is ignored here. I will discuss it in a better situation)
Disclaimer: Please note that all these are strictly personal view of mine and same must not be construed as any professional advice or suggestion etc.