According to the query of Baidu Library, the writing method of sending a letter to the supplier when the fluorine content of the paint is not up to standard is as follows:

1. Title: clearly indicate the subject at the beginning of the letter, such as "Notice on paint fluorine content not up to standard".

2. Introduction: At the beginning of the letter, briefly introduce your identity and the organization or company you represent, indicating that the other party is a supplier.

3. Problem statement: clearly point out that the fluorine content of paint does not meet the standard, and provide detailed test results or data support to let suppliers know the seriousness and specific situation of the problem.

4. Requirements: clearly express your requirements, which can be to ask the supplier to take remedial measures, provide substitute products that meet the standards or retest.

5. Time limit: Set a reasonable time limit in the letter and ask the supplier to take action or reply within this time limit.

6. Suggestions for cooperation: If possible, some suggestions or solutions can be provided to help suppliers solve problems, such as suggestions for improving production processes or providing technical support.

7. Conclusion: At the end of the letter, thank the supplier and leave your contact information so that the supplier can contact you or reply.

8. Signature: At the end of the letter, write down your name and position, and leave enough space under the signature for signature.

9. CC: If necessary, relevant departments or personnel can be copied in the letter to ensure that the problem is concerned and followed up.

10. Format and text: Make sure the format of the letter is clean and easy to read, and describe the problems and requirements in clear and objective language.