I highly recommend that you read FAR 16.702 -- Basic Agreements. Below, I have highlighted some of the relevant paragraphs of 16.702 that are the basis of my recommendation. I believe that you should add a cost reimbursement CLIN to each one of the BOA’s that you use to do this type of work so that you can maintain competition. If this work is sole-source, you must document the addition of the CLIN to the BOA with a FAR 6 Justification and Approval. Put in a cost reimbursement CLIN with a Not to Exceed Amount and explain the process for determining the items that will need to be painted. Collect data over time and evaluate the risk of turning this Cost Reimbursement CLIN into a FFP. If there is little variation in the percentage of items that will need painting, then I think it would make more sense to make this effort FFP (but only if your historical data is clear and can be used to determine a fair and reasonable price for the painting effort). I also think a Time& Material CLIN would work for this additional effort, but this contract type is very out of favor and will need approvals.
From FAR 16.702:
(c) Limitations. A basic ordering agreement shall not state or imply any agreement by the Government to place future contracts or orders with the contractor or be used in any manner to restrict competition.
(d) Orders. A contracting officer representing any Government activity listed in a basic ordering agreement may issue orders for required supplies or services covered by that agreement.
(1) Before issuing an order under a basic ordering agreement, the contracting officer shall --
(i) Obtain competition in accordance with Part 6;
(ii) If the order is being placed after competition, ensure that use of the basic ordering agreement is not prejudicial to other offerors; and
(iii) Sign or obtain any applicable justifications and approvals, and any determination and findings, and comply with other requirements in accordance with
(3) The contracting officer shall neither make any final commitment nor authorize the contractor to begin work on an order under a basic ordering agreement until prices have been established, unless the order establishes a ceiling price limiting the Government’s obligation and either --
(i) The basic ordering agreement provides adequate procedures for timely pricing of the order early in its performance period; or
(ii) The need for the supplies or services is compelling and unusually urgent (i.e., when the Government would be seriously injured, financially or otherwise, if the requirement is not met sooner than would be possible if prices were established before the work began). The contracting officer shall proceed with pricing as soon as practical. In no event shall an entire order be priced retroactively.